术语 ¶

Atomicity (via Wikipedia)

英文

Atomicity is one of the components of the ACID model, and requires that each transaction is “all or nothing”: if one part of the transaction fails, the entire transaction fails, and the database state is left unchanged. An atomic system must guarantee atomicity in each and every situation, including power failures, errors, and crashes. (via Wikipedia)

参见

Atomicity (via Wikipedia)

attached¶

随附的¶

中文

表示当前与特定 Session 关联的 ORM 对象。

参见

英文

Indicates an ORM object that is presently associated with a specific Session.

参见

backref¶

bidirectional relationship¶

双向关系¶

中文

对 relationship 系统的扩展，其中两个不同的 relationship() 对象可以相互关联，以便在任一方发生变化时在内存中协调。这两个关系最常见的构建方式是显式使用 relationship() 函数为一方指定 backref 关键字，以便另一方的 relationship() 自动创建。我们可以使用 one to many 中的示例来说明这一点:

class Department(Base):
    __tablename__ = "department"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    employees = relationship("Employee", backref="department")


class Employee(Base):
    __tablename__ = "employee"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    dep_id = Column(Integer, ForeignKey("department.id"))

backref 可以应用于任何关系，包括一对多、多对一和 many to many。

参见

英文

An extension to the relationship system whereby two distinct relationship() objects can be mutually associated with each other, such that they coordinate in memory as changes occur to either side. The most common way these two relationships are constructed is by using the relationship() function explicitly for one side and specifying the backref keyword to it so that the other relationship() is created automatically. We can illustrate this against the example we’ve used in one to many as follows:

class Department(Base):
    __tablename__ = "department"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    employees = relationship("Employee", backref="department")


class Employee(Base):
    __tablename__ = "employee"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    dep_id = Column(Integer, ForeignKey("department.id"))

A backref can be applied to any relationship, including one to many, many to one, and many to many.

参见

candidate key¶

候选键¶

中文

一个 relational algebra 术语，指的是形成行的唯一标识键的属性或属性集。一个行可以有多个候选键，每个候选键都适合作为该行的主键。表的主键始终是候选键。

参见

Candidate key (via Wikipedia)

https://www.databasestar.com/database-keys/

英文

A relational algebra term referring to an attribute or set of attributes that form a uniquely identifying key for a row. A row may have more than one candidate key, each of which is suitable for use as the primary key of that row. The primary key of a table is always a candidate key.

参见

Candidate key (via Wikipedia)

https://www.databasestar.com/database-keys/

cascade¶

级联¶

中文

一个术语，用于描述在 SQLAlchemy 中，针对特定对象执行的 ORM 持久化操作如何扩展到与该对象直接关联的其他对象。在 SQLAlchemy 中，这些对象关联是使用 relationship() 构造配置的。 relationship() 包含一个称为 relationship.cascade 的参数，该参数提供有关某些持久化操作如何级联的选项。

“级联”一词以及该系统在 SQLAlchemy 中的一般架构，无论好坏，都是从 Hibernate ORM 借用的。

参见

级联

英文

A term used in SQLAlchemy to describe how an ORM persistence action that takes place on a particular object would extend into other objects which are directly associated with that object. In SQLAlchemy, these object associations are configured using the relationship() construct. relationship() contains a parameter called relationship.cascade which provides options on how certain persistence operations may cascade.

The term “cascades” as well as the general architecture of this system in SQLAlchemy was borrowed, for better or worse, from the Hibernate ORM.

参见

级联

check constraint¶

检查约束¶

中文

一个检查约束是一个条件，用于定义在关系数据库的表中添加或更新条目时的有效数据。检查约束适用于表中的每一行。

(via Wikipedia)

可以使用标准 SQL 中的 DDL 向表添加检查约束，如下所示：

ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);

参见

CHECK constraint (via Wikipedia)

英文

A check constraint is a condition that defines valid data when adding or updating an entry in a table of a relational database. A check constraint is applied to each row in the table.

(via Wikipedia)

A check constraint can be added to a table in standard SQL using DDL like the following:

ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);

参见

CHECK constraint (via Wikipedia)

columns clause¶

中文

SELECT 语句中枚举将在结果集中返回的 SQL 表达式的部分。表达式直接跟在 SELECT 关键字后面，是一个用逗号分隔的单个表达式列表。

例如：

SELECT user_account.name, user_account.email
FROM user_account WHERE user_account.name = 'fred'

上面，列 user_account.name 和 user_account.email 是 SELECT 语句的列子句。

英文

The portion of the SELECT statement which enumerates the SQL expressions to be returned in the result set. The expressions follow the SELECT keyword directly and are a comma-separated list of individual expressions.

E.g.:

SELECT user_account.name, user_account.email
FROM user_account WHERE user_account.name = 'fred'

Above, the list of columns user_acount.name, user_account.email is the columns clause of the SELECT.

composite primary key¶

复合主键¶

中文

一个 primary key，包含多个列。特定的数据库行基于两个或多个列而不是单个值唯一。

参见

英文

A primary key that has more than one column. A particular database row is unique based on two or more columns rather than just a single value.

参见

consistency¶

一致性¶

中文

一致性是 ACID 模型的组成部分之一，确保任何事务都将数据库从一个有效状态带到另一个有效状态。写入数据库的任何数据都必须符合所有定义的规则，包括但不限于 constraints、级联、触发器及其任何组合。（via Wikipedia）

参见

Consistency (via Wikipedia)

英文

Consistency is one of the components of the ACID model, and ensures that any transaction will bring the database from one valid state to another. Any data written to the database must be valid according to all defined rules, including but not limited to constraints, cascades, triggers, and any combination thereof. (via Wikipedia)

参见

Consistency (via Wikipedia)

constraint¶

constraints¶

constrained¶

约束¶

中文

在关系数据库中建立的规则，确保数据的有效性和一致性。常见的约束形式包括 primary key constraint、foreign key constraint 和 check constraint。

英文

Rules established within a relational database that ensure the validity and consistency of data. Common forms of constraint include primary key constraint, foreign key constraint, and check constraint.

correlates¶

correlated subquery¶

相关子查询¶

correlated subqueries¶

中文

如果 subquery 依赖于封闭 SELECT 中的数据，则它是相关的。

下面，一个子查询从 email_address 表中选择聚合值 MIN(a.id)，使其针对 email_address.user_account_id 列关联 user_account.id 列的值：

SELECT user_account.name, email_address.email
    FROM user_account
    JOIN email_address ON user_account.id=email_address.user_account_id
    WHERE email_address.id = (
    SELECT MIN(a.id) FROM email_address AS a
    WHERE a.user_account_id=user_account.id
    )

上面的子查询指的是 user_account 表，该表本身不在这个嵌套查询的 FROM 子句中。相反， user_account 表是从封闭查询接收的，其中从 user_account 中选择的每一行都会导致子查询的不同执行。

在大多数情况下，相关子查询出现在直接封闭 SELECT 语句的 WHERE clause 或 columns clause 中，以及 ORDER BY 或 HAVING 子句中。

在不太常见的情况下，相关子查询可能出现在封闭 SELECT 的 FROM clause 中；在这些情况下，相关性通常是由于封闭的 SELECT 本身被封闭在另一个 SELECT 的 WHERE、ORDER BY、columns 或 HAVING 子句中，例如：

SELECT parent.id FROM parent
WHERE EXISTS (
    SELECT * FROM (
        SELECT child.id AS id, child.parent_id AS parent_id, child.pos AS pos
        FROM child
        WHERE child.parent_id = parent.id ORDER BY child.pos
    LIMIT 3)
WHERE id = 7)

不能通过 FROM 子句直接将一个 SELECT 的相关性关联到封闭查询，因为只有当封闭语句的 FROM 子句中的原始源行可用时，相关性才能进行。

英文

A subquery is correlated if it depends on data in the enclosing SELECT.

Below, a subquery selects the aggregate value MIN(a.id) from the email_address table, such that it will be invoked for each value of user_account.id, correlating the value of this column against the email_address.user_account_id column:

SELECT user_account.name, email_address.email
    FROM user_account
    JOIN email_address ON user_account.id=email_address.user_account_id
    WHERE email_address.id = (
    SELECT MIN(a.id) FROM email_address AS a
    WHERE a.user_account_id=user_account.id
    )

The above subquery refers to the user_account table, which is not itself in the FROM clause of this nested query. Instead, the user_account table is received from the enclosing query, where each row selected from user_account results in a distinct execution of the subquery.

A correlated subquery is in most cases present in the WHERE clause or columns clause of the immediately enclosing SELECT statement, as well as in the ORDER BY or HAVING clause.

In less common cases, a correlated subquery may be present in the FROM clause of an enclosing SELECT; in these cases the correlation is typically due to the enclosing SELECT itself being enclosed in the WHERE, ORDER BY, columns or HAVING clause of another SELECT, such as:

SELECT parent.id FROM parent
WHERE EXISTS (
    SELECT * FROM (
        SELECT child.id AS id, child.parent_id AS parent_id, child.pos AS pos
        FROM child
        WHERE child.parent_id = parent.id ORDER BY child.pos
    LIMIT 3)
WHERE id = 7)

Correlation from one SELECT directly to one which encloses the correlated query via its FROM clause is not possible, because the correlation can only proceed once the original source rows from the enclosing statement’s FROM clause are available.

crud¶

CRUD¶

中文

一个首字母缩略词，意思是“创建、更新、删除”。在 SQL 中，该术语指创建、修改和删除数据库中的数据的操作集，也称为 DML，通常指 INSERT、 UPDATE 和 DELETE 语句。

英文

An acronym meaning “Create, Update, Delete”. The term in SQL refers to the set of operations that create, modify and delete data from the database, also known as DML, and typically refers to the INSERT, UPDATE, and DELETE statements.

cursor¶

游标¶

中文

一种控制结构，使得能够遍历数据库中的记录。在 Python DBAPI 中，cursor 对象实际上是语句执行的起点，也是用于获取结果的接口。

参见

Cursor Objects (in pep-249)

Cursor (via Wikipedia)

英文

A control structure that enables traversal over the records in a database. In the Python DBAPI, the cursor object is in fact the starting point for statement execution as well as the interface used for fetching results.

参见

Cursor Objects (in pep-249)

Cursor (via Wikipedia)

DBAPI¶

pep-249¶

中文

DBAPI 是“Python 数据库 API 规范”（Python Database API Specification）的缩写。这是在 Python 中广泛使用的规范，用于定义所有数据库连接包的常见使用模式。DBAPI 是一个“低级”API，通常是 Python 应用程序中用于与数据库通信的最低级系统。SQLAlchemy 的 dialect 系统围绕 DBAPI 的操作构建，提供服务于特定数据库引擎的特定 DBAPI 的单个方言类；例如，create_engine() URL postgresql+psycopg2://@localhost/test 指的是 psycopg2 DBAPI/方言组合，而 URL mysql+mysqldb://@localhost/test 指的是 MySQL for Python DBAPI/方言组合。

参见

PEP 249 - Python Database API Specification v2.0

英文

DBAPI is shorthand for the phrase “Python Database API Specification”. This is a widely used specification within Python to define common usage patterns for all database connection packages. The DBAPI is a “low level” API which is typically the lowest level system used in a Python application to talk to a database. SQLAlchemy’s dialect system is constructed around the operation of the DBAPI, providing individual dialect classes which service a specific DBAPI on top of a specific database engine; for example, the create_engine() URL postgresql+psycopg2://@localhost/test refers to the psycopg2 DBAPI/dialect combination, whereas the URL mysql+mysqldb://@localhost/test refers to the MySQL for Python DBAPI/dialect combination.

参见

PEP 249 - Python Database API Specification v2.0

DDL¶

中文

一个 数据定义语言 的缩写。DDL 是 SQL 的子集，关系数据库使用它来配置表、约束和数据库模式中的其他永久对象。SQLAlchemy 提供了一个丰富的 API 用于构建和发出 DDL 表达式。

参见

英文

An acronym for Data Definition Language. DDL is the subset of SQL that relational databases use to configure tables, constraints, and other permanent objects within a database schema. SQLAlchemy provides a rich API for constructing and emitting DDL expressions.

参见

deleted¶

删除的¶

中文

这描述了对象在 Session 中可以具有的主要对象状态之一；一个已删除对象是一个以前是持久的对象，并且在刷新到数据库中发出 DELETE 语句以删除其行。对象将在会话事务提交后移动到 detached 状态；或者，如果会话事务被回滚，DELETE 将被还原，对象将返回到 persistent 状态。

参见

英文

This describes one of the major object states which an object can have within a Session; a deleted object is an object that was formerly persistent and has had a DELETE statement emitted to the database within a flush to delete its row. The object will move to the detached state once the session’s transaction is committed; alternatively, if the session’s transaction is rolled back, the DELETE is reverted and the object moves back to the persistent state.

参见

descriptor¶

描述符¶

descriptors¶

中文

在 Python 中，描述符是具有“绑定行为”的对象属性，其属性访问被描述符协议中的方法覆盖。这些方法是 __get__()、 __set__() 和 __delete__()。如果为对象定义了这些方法中的任何一个，它就被称为描述符。

在 SQLAlchemy 中，描述符被大量使用，以提供映射类的属性行为。当类被映射时，例如:

class MyClass(Base):
    __tablename__ = "foo"

    id = Column(Integer, primary_key=True)
    data = Column(String)

当 MyClass 类的定义完成时，该类将被 mapped，此时，最初作为 Column 对象的 id 和 data 属性将被 instrumentation 系统替换为 InstrumentedAttribute 的实例，这些实例是提供上述 __get__()、__set__() 和 __delete__() 方法的描述符。 InstrumentedAttribute 在类级别使用时会生成一个 SQL 表达式：

>>> print(MyClass.data==5)data = :data_1

在实例级别，它会跟踪值的变化，并且还会从数据库 lazy loads 未加载的属性：

>>> m1 = MyClass()
>>> m1.id = 5
>>> m1.data = "some data"

>>> from sqlalchemy import inspect
>>> inspect(m1).attrs.data.history.added
"some data"

英文

In Python, a descriptor is an object attribute with “binding behavior”, one whose attribute access has been overridden by methods in the descriptor protocol. Those methods are __get__(), __set__(), and __delete__(). If any of those methods are defined for an object, it is said to be a descriptor.

In SQLAlchemy, descriptors are used heavily in order to provide attribute behavior on mapped classes. When a class is mapped as such:

class MyClass(Base):
    __tablename__ = "foo"

    id = Column(Integer, primary_key=True)
    data = Column(String)

The MyClass class will be mapped when its definition is complete, at which point the id and data attributes, starting out as Column objects, will be replaced by the instrumentation system with instances of InstrumentedAttribute, which are descriptors that provide the above mentioned __get__(), __set__() and __delete__() methods. The InstrumentedAttribute will generate a SQL expression when used at the class level:

>>> print(MyClass.data==5)data = :data_1

and at the instance level, keeps track of changes to values, and also lazy loads unloaded attributes from the database:

>>> m1 = MyClass()
>>> m1.id = 5
>>> m1.data = "some data"

>>> from sqlalchemy import inspect
>>> inspect(m1).attrs.data.history.added
"some data"

detached¶

分离的¶

中文

这描述了对象在 Session 中可以具有的主要对象状态之一；一个分离对象是具有数据库标识（即主键）但未与任何会话关联的对象。以前处于 persistent 状态并被移出其会话的对象，无论是因为被删除还是拥有会话被关闭，都会移到分离状态。分离状态通常在对象在会话之间移动或在与外部对象缓存之间移动时使用。

参见

英文

This describes one of the major object states which an object can have within a Session; a detached object is an object that has a database identity (i.e. a primary key) but is not associated with any session. An object that was previously persistent and was removed from its session either because it was expunged, or the owning session was closed, moves into the detached state. The detached state is generally used when objects are being moved between sessions or when being moved to/from an external object cache.

参见

dialect¶

方言¶

中文

在 SQLAlchemy 中，“方言”是一个 Python 对象，表示允许在特定类型的数据库后端和该数据库的特定类型的 Python 驱动程序（或 DBAPI）上进行数据库操作的信息和方法。SQLAlchemy 方言是 Dialect 类的子类。

参见

Engine 配置

英文

In SQLAlchemy, the “dialect” is a Python object that represents information and methods that allow database operations to proceed on a particular kind of database backend and a particular kind of Python driver (or DBAPI) for that database. SQLAlchemy dialects are subclasses of the Dialect class.

参见

Engine 配置

discriminator¶

鉴别器¶

中文

一个结果集列，在 polymorphic 加载期间用于确定应将哪种映射类应用于特定的传入结果行。

参见

映射类继承层次结构

英文

A result-set column which is used during polymorphic loading to determine what kind of mapped class should be applied to a particular incoming result row.

参见

映射类继承层次结构

DML¶

中文

一个 数据操作语言 的缩写。DML 是 SQL 的子集，关系数据库使用它来修改表中的数据。DML 通常指三个广为人知的语句：INSERT、UPDATE 和 DELETE，也称为 CRUD （“创建、读取、更新、删除”的缩写）。

参见

DML (via Wikipedia)

DQL

英文

An acronym for Data Manipulation Language. DML is the subset of SQL that relational databases use to modify the data in tables. DML typically refers to the three widely familiar statements of INSERT, UPDATE and DELETE, otherwise known as CRUD (acronym for “Create, Read, Update, Delete”).

参见

DML (via Wikipedia)

Domain Model (via Wikipedia)

DQL

domain model¶

领域模型¶

中文

领域模型在问题解决和软件工程中是与特定问题相关的所有主题的概念模型。它描述了各种实体、它们的属性、角色和关系，以及支配问题领域的约束。

（via Wikipedia）

参见

英文

A domain model in problem solving and software engineering is a conceptual model of all the topics related to a specific problem. It describes the various entities, their attributes, roles, and relationships, plus the constraints that govern the problem domain.

(via Wikipedia)

参见

Domain Model (via Wikipedia)

DQL¶

中文

一个 数据查询语言 的缩写。DQL 是 SQL 的子集，关系数据库使用它来读取表中的数据。DQL 几乎完全指 SQL SELECT 结构作为使用的顶级 SQL 语句。

参见

DQL (via Wikipedia)

DML

英文

An acronym for Data Query Language. DQL is the subset of SQL that relational databases use to read the data in tables. DQL almost exclusively refers to the SQL SELECT construct as the top level SQL statement in use.

参见

DQL (via Wikipedia)

DML

durability¶

耐久性¶

中文

持久性是 ACID 模型的一个属性，意味着一旦事务被提交，即使在断电、崩溃或错误的情况下，它也将保持提交状态。例如，在关系数据库中，一旦一组 SQL 语句执行完毕，结果需要永久存储（即使数据库在此后立即崩溃）。（via Wikipedia）

参见

Durability (via Wikipedia)

英文

Durability is a property of the ACID model which means that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors. In a relational database, for instance, once a group of SQL statements execute, the results need to be stored permanently (even if the database crashes immediately thereafter). (via Wikipedia)

参见

Durability (via Wikipedia)

eager load¶

eager loads¶

eager loaded¶

eager loading¶

eagerly load¶

预加载¶

中文

在对象关系映射中，“预加载”指的是在对象本身从数据库加载时，属性也会被填充其数据库端的值。在 SQLAlchemy 中，”预加载” 通常指的是使用 relationship() 构造在映射之间链接的相关集合和对象实例，但也可以指加载其他列属性，通常是从与正在查询的特定表相关的其他表中加载，例如使用 inheritance 映射时。

预加载是 lazy loading 的对立面。

参见

英文

In object relational mapping, an “eager load” refers to an attribute that is populated with its database-side value at the same time as when the object itself is loaded from the database. In SQLAlchemy, the term “eager loading” usually refers to related collections and instances of objects that are linked between mappings using the relationship() construct, but can also refer to additional column attributes being loaded, often from other tables related to a particular table being queried, such as when using inheritance mappings.

Eager loading is the opposite of lazy loading.

参见

Foreign Key Constraint (via Wikipedia)

executemany¶

中文

这个术语指的是 PEP 249 DBAPI 规范的一部分，表示可以对数据库连接调用单个 SQL 语句，并使用多个参数集。具体方法称为 cursor.executemany()，它与用于单语句调用的 cursor.execute() 方法有许多行为上的不同。“executemany” 方法多次执行给定的 SQL 语句，每次使用传递的一组参数。使用 executemany 的一般理由是提高性能，其中 DBAPI 可以使用诸如预先准备语句一次的技术，或以其他方式优化多次调用相同的语句。

当 Connection.execute() 方法被使用并传递了一个参数字典列表时，SQLAlchemy 通常会自动使用 cursor.executemany() 方法；这向 SQLAlchemy Core 表明，SQL 语句和处理后的参数集应传递给 cursor.executemany()，驱动程序将为每个参数字典单独调用该语句。

cursor.executemany() 方法的一个关键限制是，当使用此方法时， cursor 未配置为返回行。对于 大多数 后端（一个显着的例外是 python-oracledb / cx_Oracle DBAPIs），这意味着像 INSERT..RETURNING 这样的语句通常不能直接与 cursor.executemany() 一起使用，因为 DBAPIs 通常不会将每次 INSERT 执行的单行聚合在一起。

为克服此限制，SQLAlchemy 从 2.0 系列开始实现了一种称为 INSERT 语句的“插入多个值”行为的替代形式的“executemany”。此功能使用 cursor.execute() 调用一个 INSERT 语句，该语句将在一次往返中处理多个参数集，从而产生与使用 cursor.executemany() 相同的效果，同时仍然支持 RETURNING。

参见

发送多个参数 - 对“executemany”的教程介绍

INSERT 语句的“插入多个值”行为 - 允许在“executemany”中使用 RETURNING 的 SQLAlchemy 功能

英文

This term refers to a part of the PEP 249 DBAPI specification indicating a single SQL statement that may be invoked against a database connection with multiple parameter sets. The specific method is known as cursor.executemany(), and it has many behavioral differences in comparison to the cursor.execute() method which is used for single-statement invocation. The “executemany” method executes the given SQL statement multiple times, once for each set of parameters passed. The general rationale for using executemany is that of improved performance, wherein the DBAPI may use techniques such as preparing the statement just once beforehand, or otherwise optimizing for invoking the same statement many times.

SQLAlchemy typically makes use of the cursor.executemany() method automatically when the Connection.execute() method is used where a list of parameter dictionaries were passed; this indicates to SQLAlchemy Core that the SQL statement and processed parameter sets should be passed to cursor.executemany(), where the statement will be invoked by the driver for each parameter dictionary individually.

A key limitation of the cursor.executemany() method as used with all known DBAPIs is that the cursor is not configured to return rows when this method is used. For most backends (a notable exception being the python-oracledb / cx_Oracle DBAPIs), this means that statements like INSERT..RETURNING typically cannot be used with cursor.executemany() directly, since DBAPIs typically do not aggregate the single row from each INSERT execution together.

To overcome this limitation, SQLAlchemy as of the 2.0 series implements an alternative form of “executemany” which is known as INSERT 语句的“插入多个值”行为. This feature makes use of cursor.execute() to invoke an INSERT statement that will proceed with multiple parameter sets in one round trip, thus producing the same effect as using cursor.executemany() while still supporting RETURNING.

参见

发送多个参数 - tutorial introduction to “executemany”

INSERT 语句的“插入多个值”行为 - SQLAlchemy feature which allows RETURNING to be used with “executemany”

expire¶

expired¶

expires¶

expiring¶

Expiring¶

过期¶

中文

在 SQLAlchemy ORM 中，指的是当 persistent 或有时是 detached 对象中的数据被清除时，下次访问对象的属性时，将发出一个 lazy load SQL 查询，以便刷新当前正在进行的事务中存储的该对象的数据。

参见

刷新/过期

英文

In the SQLAlchemy ORM, refers to when the data in a persistent or sometimes detached object is erased, such that when the object’s attributes are next accessed, a lazy load SQL query will be emitted in order to refresh the data for this object as stored in the current ongoing transaction.

参见

刷新/过期

flush¶

flushing¶

flushed¶

中文

这指的是 unit of work 用于向数据库发出更改的实际过程。在 SQLAlchemy 中，这个过程通过 Session 对象发生，通常是自动的，但也可以手动控制。

参见

刷新

英文

This refers to the actual process used by the unit of work to emit changes to a database. In SQLAlchemy this process occurs via the Session object and is usually automatic, but can also be controlled manually.

参见

刷新

foreign key constraint¶

外键约束¶

中文

一个表之间的参照约束。外键是关系表中的一个字段或一组字段，与另一个表的 candidate key 匹配。外键可用于交叉引用表。 (via Wikipedia)

可以使用标准 SQL 中的 DDL 向表添加外键约束，如下所示：

ALTER TABLE employee ADD CONSTRAINT dep_id_fk
FOREIGN KEY (employee) REFERENCES department (dep_id)

参见

英文

A referential constraint between two tables. A foreign key is a field or set of fields in a relational table that matches a candidate key of another table. The foreign key can be used to cross-reference tables. (via Wikipedia)

A foreign key constraint can be added to a table in standard SQL using DDL like the following:

ALTER TABLE employee ADD CONSTRAINT dep_id_fk
FOREIGN KEY (employee) REFERENCES department (dep_id)

参见

Foreign Key Constraint (via Wikipedia)

FROM clause¶

中文

SELECT 语句中指示行的初始来源的部分。

一个简单的 SELECT 将在其 FROM 子句中包含一个或多个表名。多个来源用逗号分隔：

SELECT user.name, address.email_address
FROM user, address
WHERE user.id=address.user_id

FROM 子句也是指定显式连接的地方。我们可以使用一个包含两个表的 JOIN 的单个 FROM 元素重写上述 SELECT：

SELECT user.name, address.email_address
FROM user JOIN address ON user.id=address.user_id

英文

The portion of the SELECT statement which indicates the initial source of rows.

A simple SELECT will feature one or more table names in its FROM clause. Multiple sources are separated by a comma:

SELECT user.name, address.email_address
FROM user, address
WHERE user.id=address.user_id

The FROM clause is also where explicit joins are specified. We can rewrite the above SELECT using a single FROM element which consists of a JOIN of the two tables:

SELECT user.name, address.email_address
FROM user JOIN address ON user.id=address.user_id

identity key¶

身份密钥¶

中文

一个与 ORM 映射对象关联的键，用于标识它们在数据库中的主键标识，以及它们在 Session identity map 中的唯一标识。

在 SQLAlchemy 中，您可以使用 inspect() API 查看 ORM 对象的标识键，以返回 InstanceState 跟踪对象，然后查看 InstanceState.key 属性:

>>> from sqlalchemy import inspect
>>> inspect(some_object).key
(<class '__main__.MyTable'>, (1,), None)

参见

identity map

英文

A key associated with ORM-mapped objects that identifies their primary key identity within the database, as well as their unique identity within a Session identity map.

In SQLAlchemy, you can view the identity key for an ORM object using the inspect() API to return the InstanceState tracking object, then looking at the InstanceState.key attribute:

>>> from sqlalchemy import inspect
>>> inspect(some_object).key
(<class '__main__.MyTable'>, (1,), None)

参见

identity map

identity map¶

身份映射¶

中文

Python 对象及其数据库标识之间的映射。身份映射是与 ORM Session 对象关联的集合，并维护每个数据库对象的单个实例，该实例以其标识为键。这种模式的优点是，针对特定数据库标识进行的所有操作都透明地协调到单个对象实例上。当将身份映射与 isolated 事务结合使用时，具有特定主键的对象引用在实际操作中可以视为实际数据库行的代理。

参见

Identity Map (via Martin Fowler)

通过主键获取 - 如何通过主键在身份映射中查找对象

英文

A mapping between Python objects and their database identities. The identity map is a collection that’s associated with an ORM Session object, and maintains a single instance of every database object keyed to its identity. The advantage to this pattern is that all operations which occur for a particular database identity are transparently coordinated onto a single object instance. When using an identity map in conjunction with an isolated transaction, having a reference to an object that’s known to have a particular primary key can be considered from a practical standpoint to be a proxy to the actual database row.

参见

Identity Map (via Martin Fowler)

通过主键获取 - how to look up an object in the identity map by primary key

insertmanyvalues¶

中文

这是一个 SQLAlchemy 特定功能，允许 INSERT 语句在单个语句中发出数千行新行，同时允许使用 RETURNING 或类似方式从语句中内联返回服务器生成的值，以进行性能优化。该功能旨在为选定的后端透明地提供，但确实提供了一些配置选项。有关此功能的完整描述，请参阅 INSERT 语句的“插入多个值”行为部分。

参见

INSERT 语句的“插入多个值”行为

英文

This refers to a SQLAlchemy-specific feature which allows INSERT statements to emit thousands of new rows within a single statement while at the same time allowing server generated values to be returned inline from the statement using RETURNING or similar, for performance optimization purposes. The feature is intended to be transparently available for selected backends, but does offer some configurational options. See the section INSERT 语句的“插入多个值”行为 for a full description of this feature.

参见

INSERT 语句的“插入多个值”行为

instrumentation¶

instrumented¶

仪器化¶

instrumenting¶

中文

Instrumentation 是指增强特定类的功能和属性集的过程。理想情况下，类的行为应保持接近普通类，只是提供了额外的行为和功能。SQLAlchemy 的 mapping 过程除了其他功能外，还会向映射类添加数据库启用的 descriptors，每个描述符代表一个特定的数据库列或与相关类的关系。

英文

Instrumentation refers to the process of augmenting the functionality and attribute set of a particular class. Ideally, the behavior of the class should remain close to a regular class, except that additional behaviors and features are made available. The SQLAlchemy mapping process, among other things, adds database-enabled descriptors to a mapped class each of which represents a particular database column or relationship to a related class.

isolation¶

isolated¶

isolation level¶

隔离级别¶

中文

隔离性是 ACID 模型的组成部分之一，确保事务的并发执行结果是一个系统状态，该状态将是事务按顺序执行时获得的状态，即一个接一个地执行。每个事务必须在完全隔离的情况下执行，即如果 T1 和 T2 并发执行，则每个事务应保持独立。（via Wikipedia）

参见

Isolation (via Wikipedia)

英文

The isolation property of the ACID model ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially, i.e. one after the other. Each transaction must execute in total isolation i.e. if T1 and T2 execute concurrently then each should remain independent of the other. (via Wikipedia)

参见

Isolation (via Wikipedia)

lazy initialization¶

延迟初始化¶

中文

一种延迟某些初始化操作的策略，例如创建对象、填充数据或建立与其他服务的连接，直到需要这些资源时才进行。

参见

Lazy initialization (via Wikipedia)

英文

A tactic of delaying some initialization action, such as creating objects, populating data, or establishing connectivity to other services, until those resources are required.

参见

Lazy initialization (via Wikipedia)

lazy load¶

lazy loads¶

lazy loaded¶

lazy loading¶

延迟加载¶

中文

在对象关系映射中，“延迟加载”指的是在某段时间内（通常是对象首次加载时）属性不包含其数据库端的值。相反，属性会接收到一个 memoization，使其在首次使用时访问数据库并加载数据。使用这种模式，有时可以减少对象获取的复杂性和时间，因为不需要立即处理相关表的属性。

延迟加载是 eager loading 的对立面。

在 SQLAlchemy 中，延迟加载是 ORM 的一个关键特性，适用于用户定义类上 mapped 的属性。当访问引用数据库列或相关对象的属性时，如果没有加载值，ORM 将使用当前对象所关联的 Session 在 persistent 状态下，并在当前事务上发出一个 SELECT 语句，如果没有进行中的事务，则启动一个新事务。如果对象处于 detached 状态且未与任何 Session 关联，这被视为错误状态并引发 informative exception。

参见

Lazy Load (via Martin Fowler)

N plus one problem

列加载选项 - 包含有关 ORM 映射列的延迟加载信息

关系加载技术 - 包含有关 ORM 相关对象的延迟加载信息

使用 AsyncSession 时防止隐式 IO - 在使用异步 I/O (asyncio) 扩展时避免延迟加载的提示

英文

In object relational mapping, a “lazy load” refers to an attribute that does not contain its database-side value for some period of time, typically when the object is first loaded. Instead, the attribute receives a memoization that causes it to go out to the database and load its data when it’s first used. Using this pattern, the complexity and time spent within object fetches can sometimes be reduced, in that attributes for related tables don’t need to be addressed immediately.

Lazy loading is the opposite of eager loading.

Within SQLAlchemy, lazy loading is a key feature of the ORM, and applies to attributes which are mapped on a user-defined class. When attributes that refer to database columns or related objects are accessed, for which no loaded value is present, the ORM makes use of the Session for which the current object is associated with in the persistent state, and emits a SELECT statement on the current transaction, starting a new transaction if one was not in progress. If the object is in the detached state and not associated with any Session, this is considered to be an error state and an informative exception is raised.

参见

Lazy Load (via Martin Fowler)

N plus one problem

列加载选项 - includes information on lazy loading of ORM mapped columns

关系加载技术 - includes information on lazy loading of ORM related objects

使用 AsyncSession 时防止隐式 IO - tips on avoiding lazy loading when using the 异步 I/O (asyncio) extension

many to many¶

多对多¶

中文

一种 sqlalchemy.orm.relationship() 样式，通过中间表将两个表链接在一起。使用此配置，左侧的任意数量的行可以引用右侧的任意数量的行，反之亦然。

一个将员工与项目关联的模式：

CREATE TABLE employee (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE project (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE employee_project (
    employee_id INTEGER PRIMARY KEY,
    project_id INTEGER PRIMARY KEY,
    FOREIGN KEY (employee_id) REFERENCES employee(id),
    FOREIGN KEY (project_id) REFERENCES project(id)
)

如上所述， employee_project 表是多对多表，自然形成由每个相关表的主键组成的复合主键。

在 SQLAlchemy 中，sqlalchemy.orm.relationship() 函数可以以大多数透明的方式表示这种关系，其中多对多表使用简单的表元数据指定:

class Employee(Base):
    __tablename__ = "employee"

    id = Column(Integer, primary_key=True)
    name = Column(String(30))

    projects = relationship(
        "Project",
        secondary=Table(
            "employee_project",
            Base.metadata,
            Column("employee_id", Integer, ForeignKey("employee.id"), primary_key=True),
            Column("project_id", Integer, ForeignKey("project.id"), primary_key=True),
        ),
        backref="employees",
    )


class Project(Base):
    __tablename__ = "project"

    id = Column(Integer, primary_key=True)
    name = Column(String(30))

如上所述，定义了 Employee.projects 和反向引用的 Project.employees 集合:

proj = Project(name="Client A")

emp1 = Employee(name="emp1")
emp2 = Employee(name="emp2")

proj.employees.extend([emp1, emp2])

参见

association relationship

英文

A style of sqlalchemy.orm.relationship() which links two tables together via an intermediary table in the middle. Using this configuration, any number of rows on the left side may refer to any number of rows on the right, and vice versa.

A schema where employees can be associated with projects:

CREATE TABLE employee (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE project (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE employee_project (
    employee_id INTEGER PRIMARY KEY,
    project_id INTEGER PRIMARY KEY,
    FOREIGN KEY employee_id REFERENCES employee(id),
    FOREIGN KEY project_id REFERENCES project(id)
)

Above, the employee_project table is the many-to-many table, which naturally forms a composite primary key consisting of the primary key from each related table.

In SQLAlchemy, the sqlalchemy.orm.relationship() function can represent this style of relationship in a mostly transparent fashion, where the many-to-many table is specified using plain table metadata:

class Employee(Base):
    __tablename__ = "employee"

    id = Column(Integer, primary_key=True)
    name = Column(String(30))

    projects = relationship(
        "Project",
        secondary=Table(
            "employee_project",
            Base.metadata,
            Column("employee_id", Integer, ForeignKey("employee.id"), primary_key=True),
            Column("project_id", Integer, ForeignKey("project.id"), primary_key=True),
        ),
        backref="employees",
    )


class Project(Base):
    __tablename__ = "project"

    id = Column(Integer, primary_key=True)
    name = Column(String(30))

Above, the Employee.projects and back-referencing Project.employees collections are defined:

proj = Project(name="Client A")

emp1 = Employee(name="emp1")
emp2 = Employee(name="emp2")

proj.employees.extend([emp1, emp2])

参见

association relationship

many to one¶

多对一¶

中文

一种 relationship() 样式，它将父映射器表中的外键链接到相关表的主键。每个父对象可以引用零个或一个相关对象。

相关对象反过来将与引用它们的任意数量的父对象具有隐式或显式的 one to many 关系。

一个多对一模式的示例（请注意，这与 one to many 模式相同）：

CREATE TABLE department (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE employee (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30),
    dep_id INTEGER REFERENCES department(id)
)

employee 到 department 的关系是多对一，因为许多员工记录可以与一个部门关联。一个 SQLAlchemy 映射可能如下所示:

class Department(Base):
    __tablename__ = "department"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))


class Employee(Base):
    __tablename__ = "employee"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    dep_id = Column(Integer, ForeignKey("department.id"))
    department = relationship("Department")

参见

英文

A style of relationship() which links a foreign key in the parent mapper’s table to the primary key of a related table. Each parent object can then refer to exactly zero or one related object.

The related objects in turn will have an implicit or explicit one to many relationship to any number of parent objects that refer to them.

An example many to one schema (which, note, is identical to the one to many schema):

CREATE TABLE department (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE employee (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30),
    dep_id INTEGER REFERENCES department(id)
)

The relationship from employee to department is many to one, since many employee records can be associated with a single department. A SQLAlchemy mapping might look like:

class Department(Base):
    __tablename__ = "department"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))


class Employee(Base):
    __tablename__ = "employee"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    dep_id = Column(Integer, ForeignKey("department.id"))
    department = relationship("Department")

参见

Marshalling (via Wikipedia)

mapping¶

mapped¶

mapped class¶

ORM mapped class¶

映射类¶

ORM映射类¶

中文

我们说一个类是“映射的”，当它与 Mapper 类的实例关联时。这个过程将类与数据库表或其他 selectable 构造关联在一起，以便可以使用 Session 持久化和加载它的实例。

参见

ORM 映射类概述

英文

We say a class is “mapped” when it has been associated with an instance of the Mapper class. This process associates the class with a database table or other selectable construct, so that instances of it can be persisted and loaded using a Session.

参见

ORM 映射类概述

marshalling¶

数据编组¶

data marshalling¶

中文

将对象的内存表示转换为适合存储或传输到系统另一部分的数据格式的过程，当数据必须在计算机程序的不同部分之间或从一个程序移动到另一个程序时。就 SQLAlchemy 而言，我们通常需要将数据“编组”成适合传递到关系数据库的格式。

参见

增强现有类型 - SQLAlchemy 的 TypeDecorator 通常用于将数据发送到数据库进行 INSERT 和 UPDATE 语句时的数据编组，以及使用 SELECT 语句检索数据时的“解组”。

英文

The process of transforming the memory representation of an object to a data format suitable for storage or transmission to another part of a system, when data must be moved between different parts of a computer program or from one program to another. In terms of SQLAlchemy, we often need to “marshal” data into a format appropriate for passing into the relational database.

参见

Marshalling (via Wikipedia)

增强现有类型 - SQLAlchemy’s TypeDecorator is commonly used for data marshalling as data is sent into the database for INSERT and UPDATE statements, and “unmarshalling” data as it is retrieved using SELECT statements.

metadata¶

元数据¶

database metadata¶

数据库元数据¶

table metadata¶

表元数据¶

中文

术语 “元数据” 通常指“ 描述数据的数据”；本身代表某种其他数据格式和/或结构的数据。在 SQLAlchemy 中，术语 “元数据” 通常指 MetaData 构造，它是关于表、列、约束和其他可能存在于特定数据库中的 DDL 对象的信息集合。

参见

Metadata Mapping (via Martin Fowler)

使用数据库元数据 - 在 SQLAlchemy 统一教程中

英文

The term “metadata” generally refers to “data that describes data”; data that itself represents the format and/or structure of some other kind of data. In SQLAlchemy, the term “metadata” typically refers to the MetaData construct, which is a collection of information about the tables, columns, constraints, and other DDL objects that may exist in a particular database.

参见

Metadata Mapping (via Martin Fowler)

使用数据库元数据 - in the SQLAlchemy 统一教程

method chaining¶

方法链¶

generative¶

生成式¶

中文

“方法链”，在 SQLAlchemy 文档中称为“生成式”，是一种面向对象的技术，通过在对象上调用方法来构建对象的状态。对象具有任意数量的方法，每个方法返回一个具有附加状态的新对象（或在某些情况下相同的对象）。

在 SQLAlchemy 中，最常使用方法链的两个对象是 Select 对象和 Query 对象。例如，可以通过调用 Select.where() 和 Select.order_by() 方法，为 Select 对象的 WHERE 子句分配两个表达式以及一个 ORDER BY 子句:

stmt = (
    select(user.c.name)
    .where(user.c.id > 5)
    .where(user.c.name.like("e%"))
    .order_by(user.c.name)
)

上述每个方法调用都会返回原始 Select 对象的副本，并添加附加的限定符。

英文

“Method chaining”, referred to within SQLAlchemy documentation as “generative”, is an object-oriented technique whereby the state of an object is constructed by calling methods on the object. The object features any number of methods, each of which return a new object (or in some cases the same object) with additional state added to the object.

The two SQLAlchemy objects that make the most use of method chaining are the Select object and the Query object. For example, a Select object can be assigned two expressions to its WHERE clause as well as an ORDER BY clause by calling upon the Select.where() and Select.order_by() methods:

stmt = (
    select(user.c.name)
    .where(user.c.id > 5)
    .where(user.c.name.like("e%"))
    .order_by(user.c.name)
)

Each method call above returns a copy of the original Select object with additional qualifiers added.

N plus one problem¶

N plus one¶

中文

N+1 问题是 lazy load 模式的一个常见副作用，其中应用程序希望遍历对象结果集中每个成员的相关属性或集合，而该属性或集合设置为通过延迟加载模式进行加载。最终结果是发出一个 SELECT 语句来加载父对象的初始结果集；然后，当应用程序遍历每个成员时，会为每个成员发出一个额外的 SELECT 语句，以加载该成员的相关属性或集合。最终结果是，对于 N 个父对象的结果集，将发出 N+1 个 SELECT 语句。

N+1 问题可以通过 eager loading 来缓解。

参见

加载器策略

英文

The N plus one problem is a common side effect of the lazy load pattern, whereby an application wishes to iterate through a related attribute or collection on each member of a result set of objects, where that attribute or collection is set to be loaded via the lazy load pattern. The net result is that a SELECT statement is emitted to load the initial result set of parent objects; then, as the application iterates through each member, an additional SELECT statement is emitted for each member in order to load the related attribute or collection for that member. The end result is that for a result set of N parent objects, there will be N + 1 SELECT statements emitted.

The N plus one problem is alleviated using eager loading.

参见

加载器策略

one to many¶

一对多¶

中文

一种 relationship() 样式，它将父映射器表的主键链接到相关表的外键。每个唯一的父对象可以引用零个或多个唯一的相关对象。

相关对象反过来将与其父对象有隐式或显式的 many to one 关系。

一个一对多模式的示例（请注意，这与 many to one 模式相同）：

CREATE TABLE department (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE employee (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30),
    dep_id INTEGER REFERENCES department(id)
)

department 到 employee 的关系是一对多，因为许多员工记录可以与一个部门关联。一个 SQLAlchemy 映射可能如下所示:

class Department(Base):
    __tablename__ = "department"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    employees = relationship("Employee")


class Employee(Base):
    __tablename__ = "employee"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    dep_id = Column(Integer, ForeignKey("department.id"))

参见

英文

A style of relationship() which links the primary key of the parent mapper’s table to the foreign key of a related table. Each unique parent object can then refer to zero or more unique related objects.

The related objects in turn will have an implicit or explicit many to one relationship to their parent object.

An example one to many schema (which, note, is identical to the many to one schema):

CREATE TABLE department (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30)
)

CREATE TABLE employee (
    id INTEGER PRIMARY KEY,
    name VARCHAR(30),
    dep_id INTEGER REFERENCES department(id)
)

The relationship from department to employee is one to many, since many employee records can be associated with a single department. A SQLAlchemy mapping might look like:

class Department(Base):
    __tablename__ = "department"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    employees = relationship("Employee")


class Employee(Base):
    __tablename__ = "employee"
    id = Column(Integer, primary_key=True)
    name = Column(String(30))
    dep_id = Column(Integer, ForeignKey("department.id"))

参见

ORM-注解¶

ORM-annotated¶

annotations¶

中文

术语“ORM-annotated”指的是 SQLAlchemy 的一个内部方面，其中一个核心对象（例如 Column 对象）可以携带额外的运行时信息，该信息标记它属于特定的 ORM 映射。这个术语不应与常用短语“类型注释”混淆，后者指的是用于静态类型的 Python 源代码“类型提示”，如 PEP 484 中介绍的那样。

SQLAlchemy 的大多数文档代码示例都格式化为一个关于“Annotated Example”或“Non-annotated Example”的小注释。这是指示例是否为 PEP 484 注释，与 SQLAlchemy 的“ORM-annotated”概念无关。

当文档中出现“ORM-annotated”一词时，它指的是核心 SQL 表达式对象，如 Table、 Column 和 Select 对象，这些对象源自或指向源自一个或多个 ORM 映射的子元素，因此在传递给 ORM 方法（例如 Session.execute()）时将具有 ORM 特定的解释和/或行为。例如，当我们从 ORM 映射构建 Select 对象时，如 ORM Tutorial 中说明的 User 类

>>> stmt = select(User)

上面的 Select 的内部状态引用了 User 映射到的 Table。User 类本身并未立即引用。这就是 Select 构造与核心级别过程兼容的方式（请注意，Select 的 ._raw_columns 成员是私有的，不应由最终用户代码访问）:

>>> stmt._raw_columns
[Table('user_account', MetaData(), Column('id', Integer(), ...)]

然而，当我们将 Select 传递给 ORM Session 时，与对象间接关联的 ORM 实体用于在 ORM 上下文中解释此 Select。实际的“ORM 注释”可以在另一个私有变量 ._annotations 中看到:

>>> stmt._raw_columns[0]._annotations
immutabledict({
'entity_namespace': <Mapper at 0x7f4dd8098c10; User>,
'parententity': <Mapper at 0x7f4dd8098c10; User>,
'parentmapper': <Mapper at 0x7f4dd8098c10; User>
})

因此我们将 stmt 称为 ORM-annotated select() 对象。这是一个 Select 语句，包含额外的信息，当传递给诸如 Session.execute() 之类的方法时，这些信息将使其在 ORM 特定的方式中进行解释。

英文

The phrase “ORM-annotated” refers to an internal aspect of SQLAlchemy, where a Core object such as a Column object can carry along additional runtime information that marks it as belonging to a particular ORM mapping. The term should not be confused with the common phrase “type annotation”, which refers to Python source code “type hints” used for static typing as introduced at PEP 484.

Most of SQLAlchemy’s documented code examples are formatted with a small note regarding “Annotated Example” or “Non-annotated Example”. This refers to whether or not the example is PEP 484 annotated, and is not related to the SQLAlchemy concept of “ORM-annotated”.

When the phrase “ORM-annotated” appears in documentation, it is referring to Core SQL expression objects such as Table, Column, and Select objects, which originate from, or refer to sub-elements that originate from, one or more ORM mappings, and therefore will have ORM-specific interpretations and/or behaviors when passed to ORM methods such as Session.execute(). For example, when we construct a Select object from an ORM mapping, such as the User class illustrated in the ORM Tutorial:

>>> stmt = select(User)

The internal state of the above Select refers to the Table to which User is mapped. The User class itself is not immediately referenced. This is how the Select construct remains compatible with Core-level processes (note that the ._raw_columns member of Select is private and should not be accessed by end-user code):

>>> stmt._raw_columns
[Table('user_account', MetaData(), Column('id', Integer(), ...)]

However, when our Select is passed along to an ORM Session, the ORM entities that are indirectly associated with the object are used to interpret this Select in an ORM context. The actual “ORM annotations” can be seen in another private variable ._annotations:

>>> stmt._raw_columns[0]._annotations
immutabledict({
'entity_namespace': <Mapper at 0x7f4dd8098c10; User>,
'parententity': <Mapper at 0x7f4dd8098c10; User>,
'parentmapper': <Mapper at 0x7f4dd8098c10; User>
})

Therefore we refer to stmt as an ORM-annotated select() object. It’s a Select statement that contains additional information that will cause it to be interpreted in an ORM-specific way when passed to methods like Session.execute().

pending¶

待办的¶

中文

这描述了对象在 Session 中可以具有的主要对象状态之一；一个挂起对象是一个没有任何数据库标识的新对象，但最近已与会话关联。当会话发出刷新并插入行时，对象会移动到 persistent 状态。

参见

英文

This describes one of the major object states which an object can have within a Session; a pending object is a new object that doesn’t have any database identity, but has been recently associated with a session. When the session emits a flush and the row is inserted, the object moves to the persistent state.

参见

persistent¶

持久的¶

中文

这描述了对象在 Session 中可以具有的主要对象状态之一；一个持久对象是具有数据库标识（即主键）并且当前与会话关联的对象。任何先前处于 pending 状态并且现在已插入的对象，以及任何由会话从数据库加载的对象，都是处于持久状态。当一个持久对象从会话中移除时，它被称为 detached。

参见

英文

This describes one of the major object states which an object can have within a Session; a persistent object is an object that has a database identity (i.e. a primary key) and is currently associated with a session. Any object that was previously pending and has now been inserted is in the persistent state, as is any object that’s been loaded by the session from the database. When a persistent object is removed from a session, it is known as detached.

参见

ORM Query Unified with Core Select

plugin¶

plugin-enabled¶

plugin-specific¶

中文

“plugin-enabled” 或 “plugin-specific” 通常表示 SQLAlchemy Core 中的一个函数或方法在 ORM 上下文中使用时会表现得不同。

SQLAlchemy 允许核心构造（例如 Select 对象）参与“插件”系统，该系统可以向对象注入默认情况下不存在的其他行为和功能。

具体来说，主要的“插件”是“orm”插件，它是 SQLAlchemy ORM 利用核心构造来组成和执行返回 ORM 结果的 SQL 查询的系统基础。

参见

英文

“plugin-enabled” or “plugin-specific” generally indicates a function or method in SQLAlchemy Core which will behave differently when used in an ORM context.

SQLAlchemy allows Core constructs such as Select objects to participate in a “plugin” system, which can inject additional behaviors and features into the object that are not present by default.

Specifically, the primary “plugin” is the “orm” plugin, which is at the base of the system that the SQLAlchemy ORM makes use of Core constructs in order to compose and execute SQL queries that return ORM results.

参见

ORM Query Unified with Core Select

polymorphic¶

polymorphically¶

多态的¶

中文

指的是一次处理多种类型的函数。在 SQLAlchemy 中，该术语通常应用于 ORM 映射类的概念，其中查询操作将根据结果集中信息返回不同的子类，通常通过检查结果中称为 discriminator 的特定列的值来实现。

在 SQLAlchemy 中，多态加载意味着使用三种不同方案中的一种或组合来映射类的层次结构：“joined”、“single”和“concrete”。部分映射类继承层次结构完整描述了继承映射。

英文

Refers to a function that handles several types at once. In SQLAlchemy, the term is usually applied to the concept of an ORM mapped class whereby a query operation will return different subclasses based on information in the result set, typically by checking the value of a particular column in the result known as the discriminator.

Polymorphic loading in SQLAlchemy implies that a one or a combination of three different schemes are used to map a hierarchy of classes; “joined”, “single”, and “concrete”. The section 映射类继承层次结构 describes inheritance mapping fully.

primary key¶

主键¶

primary key constraint¶

主键约束¶

中文

一种 constraint，唯一定义表中每行的特征。主键必须由任何其他行都不能复制的特征组成。主键可以由单个属性或多个属性组合而成。 (via Wikipedia)

表的主键通常（但并非总是）在 CREATE TABLE DDL 中定义：

CREATE TABLE employee (
        emp_id INTEGER,
        emp_name VARCHAR(30),
        dep_id INTEGER,
        PRIMARY KEY (emp_id)
)

参见

composite primary key

Primary key (via Wikipedia)

英文

A constraint that uniquely defines the characteristics of each row in a table. The primary key has to consist of characteristics that cannot be duplicated by any other row. The primary key may consist of a single attribute or multiple attributes in combination. (via Wikipedia)

The primary key of a table is typically, though not always, defined within the CREATE TABLE DDL:

CREATE TABLE employee (
        emp_id INTEGER,
        emp_name VARCHAR(30),
        dep_id INTEGER,
        PRIMARY KEY (emp_id)
)

参见

composite primary key

Primary key (via Wikipedia)

read committed¶

读已提交¶

中文

四种数据库 isolation 级别之一，read committed 确保事务不会暴露给其他并发事务中尚未提交的任何数据，从而防止所谓的“脏读”。然而，在 read committed 下可能会有不可重复读，这意味着如果另一个事务已提交更改，则第二次读取行中的数据可能会更改。

英文

One of the four database isolation levels, read committed features that the transaction will not be exposed to any data from other concurrent transactions that has not been committed yet, preventing so-called “dirty reads”. However, under read committed there can be non-repeatable reads, meaning data in a row may change when read a second time if another transaction has committed changes.

read uncommitted¶

读取未提交¶

中文

四种数据库 isolation 级别之一，read uncommitted 的特性是事务中的数据更改在事务提交之前不会永久生效。然而，在 read uncommitted 中，可能在另一个事务的范围内查看未在其他事务中提交的数据；这些被称为“脏读”。

英文

One of the four database isolation levels, read uncommitted features that changes made to database data within a transaction will not become permanent until the transaction is committed. However, within read uncommitted, it may be possible for data that is not committed in other transactions to be viewable within the scope of another transaction; these are known as “dirty reads”.

registry¶

注册¶

中文

一个对象，通常是全局可访问的，包含一些程序状态的长期信息，这些信息通常对程序的许多部分都很有用。

参见

Registry (via Martin Fowler)

英文

An object, typically globally accessible, that contains long-lived information about some program state that is generally useful to many parts of a program.

参见

Registry (via Martin Fowler)

relationship¶

relationships¶

中文

两个映射类之间的连接单元，对应于数据库中两个表之间的某种关系。

该关系使用 SQLAlchemy 函数 relationship() 定义。一旦创建，SQLAlchemy 会检查涉及的参数和底层映射，以将关系分类为以下三种类型之一：one to many、many to one 或 many to many。通过这种分类，关系结构处理数据库中适当链接的持久化任务，以响应内存中的对象关联，以及根据数据库中的当前链接将对象引用和集合加载到内存中的任务。

参见

关系配置

英文

A connecting unit between two mapped classes, corresponding to some relationship between the two tables in the database.

The relationship is defined using the SQLAlchemy function relationship(). Once created, SQLAlchemy inspects the arguments and underlying mappings involved in order to classify the relationship as one of three types: one to many, many to one, or many to many. With this classification, the relationship construct handles the task of persisting the appropriate linkages in the database in response to in-memory object associations, as well as the job of loading object references and collections into memory based on the current linkages in the database.

参见

关系配置

release¶

releases¶

released¶

释放¶

中文

在 SQLAlchemy 中，术语“释放”是指结束特定数据库连接的使用过程。SQLAlchemy 具有连接池的使用功能，可以配置数据库连接的生命周期。当使用池化连接时，“关闭”它的过程，即调用 connection.close() 语句，可能会将连接返回到现有池中，也可能会关闭该连接所引用的底层 TCP/IP 连接——哪种情况发生取决于配置以及池的当前状态。因此，我们使用术语 released，意为“在我们使用完连接后，做任何你做的事情”。

该术语有时会在短语“释放事务资源”中使用，以更明确地表示我们实际上“释放”的是连接上已累积的任何事务状态。在大多数情况下，从表中选择、发出更新等过程会在该连接上获取 isolated 状态以及潜在的行或表锁。这种状态都是特定事务的本地状态，当我们发出回滚时会被释放。连接池的一个重要特性是，当我们将连接返回到池中时，也会调用 DBAPI 的 connection.rollback() 方法，因此当连接被设置为再次使用时，它处于“干净”的状态，不再引用之前的一系列操作。

参见

连接池

英文

In the context of SQLAlchemy, the term “released” refers to the process of ending the usage of a particular database connection. SQLAlchemy features the usage of connection pools, which allows configurability as to the lifespan of database connections. When using a pooled connection, the process of “closing” it, i.e. invoking a statement like connection.close(), may have the effect of the connection being returned to an existing pool, or it may have the effect of actually shutting down the underlying TCP/IP connection referred to by that connection - which one takes place depends on configuration as well as the current state of the pool. So we used the term released instead, to mean “do whatever it is you do with connections when we’re done using them”.

The term will sometimes be used in the phrase, “release transactional resources”, to indicate more explicitly that what we are actually “releasing” is any transactional state which as accumulated upon the connection. In most situations, the process of selecting from tables, emitting updates, etc. acquires isolated state upon that connection as well as potential row or table locks. This state is all local to a particular transaction on the connection, and is released when we emit a rollback. An important feature of the connection pool is that when we return a connection to the pool, the connection.rollback() method of the DBAPI is called as well, so that as the connection is set up to be used again, it’s in a “clean” state with no references held to the previous series of operations.

参见

连接池

repeatable read¶

可重复读¶

中文

四种数据库 isolation 级别之一，repeatable read 具有 read committed 的所有隔离特性，此外，还具有在事务中读取的任何特定行在该事务期间保证不受任何后续外部更改（即来自其他并发 UPDATE 语句）的影响。

英文

One of the four database isolation levels, repeatable read features all of the isolation of read committed, and additionally features that any particular row that is read within a transaction is guaranteed from that point to not have any subsequent external changes in value (i.e. from other concurrent UPDATE statements) for the duration of that transaction.

RETURNING¶

中文

这是某些后端以各种形式提供的非SQL标准子句，提供在执行 INSERT、UPDATE 或 DELETE 语句时返回结果集的服务。可以返回匹配行的任何列集，就像它们是由 SELECT 语句生成的一样。

RETURNING 子句为常见的更新/选择场景提供了显著的性能提升，包括在创建时检索内联或默认生成的主键值和默认值，以及以原子方式获取服务器生成的默认值。

PostgreSQL 中的 RETURNING 示例如下所示：

INSERT INTO user_account (name) VALUES ('new name') RETURNING id, timestamp

如上所述，INSERT 语句在执行时将提供一个结果集，其中包括列 user_account.id 和 user_account.timestamp 的值，上述值应作为默认值生成，因为它们未包含在其他地方（但请注意，任何列或 SQL 表达式序列都可以放入 RETURNING 中，而不仅仅是默认值列）。

当前支持 RETURNING 或类似构造的后端有 PostgreSQL、SQL Server、Oracle Database 和 Firebird。PostgreSQL 和 Firebird 的实现通常是全功能的，而 SQL Server 和 Oracle Database 的实现有一些注意事项。在 SQL Server 上，该子句在 INSERT 和 UPDATE 语句中称为”OUTPUT INSERTED”，在 DELETE 语句中称为”OUTPUT DELETED”；关键注意事项是触发器与此关键字不支持。在 Oracle Database 中，它被称为”RETURNING…INTO”，并且要求将值放入 OUT 参数中，这不仅使语法笨拙，而且一次只能用于一行。

SQLAlchemy 的 UpdateBase.returning() 系统在这些后端的 RETURNING 系统之上提供了一个抽象层，以提供一致的列返回接口。ORM 还包括许多在可用时使用 RETURNING 的优化。

英文

This is a non-SQL standard clause provided in various forms by certain backends, which provides the service of returning a result set upon execution of an INSERT, UPDATE or DELETE statement. Any set of columns from the matched rows can be returned, as though they were produced from a SELECT statement.

The RETURNING clause provides both a dramatic performance boost to common update/select scenarios, including retrieval of inline- or default- generated primary key values and defaults at the moment they were created, as well as a way to get at server-generated default values in an atomic way.

An example of RETURNING, idiomatic to PostgreSQL, looks like:

INSERT INTO user_account (name) VALUES ('new name') RETURNING id, timestamp

Above, the INSERT statement will provide upon execution a result set which includes the values of the columns user_account.id and user_account.timestamp, which above should have been generated as default values as they are not included otherwise (but note any series of columns or SQL expressions can be placed into RETURNING, not just default-value columns).

The backends that currently support RETURNING or a similar construct are PostgreSQL, SQL Server, Oracle Database, and Firebird. The PostgreSQL and Firebird implementations are generally full featured, whereas the implementations of SQL Server and Oracle Database have caveats. On SQL Server, the clause is known as “OUTPUT INSERTED” for INSERT and UPDATE statements and “OUTPUT DELETED” for DELETE statements; the key caveat is that triggers are not supported in conjunction with this keyword. In Oracle Database, it is known as “RETURNING…INTO”, and requires that the value be placed into an OUT parameter, meaning not only is the syntax awkward, but it can also only be used for one row at a time.

SQLAlchemy’s UpdateBase.returning() system provides a layer of abstraction on top of the RETURNING systems of these backends to provide a consistent interface for returning columns. The ORM also includes many optimizations that make use of RETURNING when available.

serializable¶

可序列化¶

中文

四种数据库 isolation 级别之一，serializable 具有 repeatable read 的所有隔离特性，并且在基于锁的方法中保证不会发生所谓的“幻读”；这意味着在其他事务范围内 INSERT 或 DELETE 的行在此事务中将不可检测。在此事务中读取的行保证继续存在，而不存在的行保证不会因另一个事务的插入而出现。

Serializable 隔离通常依赖于行或行范围的锁定来实现此效果，这可能会增加死锁的可能性并降低性能。还有一些非锁定的方案，但这些方案必然依赖于在检测到写冲突时拒绝事务。

英文

One of the four database isolation levels, serializable features all of the isolation of repeatable read, and additionally within a lock-based approach guarantees that so-called “phantom reads” cannot occur; this means that rows which are INSERTed or DELETEd within the scope of other transactions will not be detectable within this transaction. A row that is read within this transaction is guaranteed to continue existing, and a row that does not exist is guaranteed that it cannot appear of inserted from another transaction.

Serializable isolation typically relies upon locking of rows or ranges of rows in order to achieve this effect and can increase the chance of deadlocks and degrade performance. There are also non-lock based schemes however these necessarily rely upon rejecting transactions if write collisions are detected.

Session¶

中文

ORM 数据库操作的容器或范围。会话从数据库加载实例，跟踪映射实例的更改，并在刷新时在一个工作单元中持久化更改。

参见

使用会话

英文

The container or scope for ORM database operations. Sessions load instances from the database, track changes to mapped instances and persist changes in a single unit of work when flushed.

参见

使用会话

subquery¶

子查询¶

scalar subquery¶

标量子查询¶

中文

指嵌入在封闭的 SELECT 语句中的 SELECT 语句。

子查询有两种一般形式，一种称为“标量选择”，它必须返回一行和一列，另一种形式作为“派生表”并用作另一个选择的 FROM 子句的行来源。标量选择可以放在封闭选择的 WHERE clause、columns clause、ORDER BY 子句或 HAVING 子句中，而派生表形式可以放在封闭 SELECT 的 FROM 子句中。

示例：

放在封闭 SELECT 的 columns clause 中的标量子查询。在此示例中，子查询是 correlated subquery，因为它选择的部分行是通过封闭语句给出的。
SELECT id, (SELECT name FROM address WHERE address.user_id=user.id) FROM user
放在封闭 SELECT 的 WHERE clause 中的标量子查询。在此示例中，该子查询未关联，因为它选择了一个固定结果。
SELECT id, name FROM user WHERE status=(SELECT status_id FROM status_code WHERE code=’C’)
放在封闭 SELECT 的 FROM clause 中的派生表子查询。此类子查询几乎总是给出别名。
SELECT user.id, user.name, ad_subq.email_address FROM

user JOIN (select user_id, email_address FROM address WHERE address_type=’Q’) AS ad_subq ON user.id = ad_subq.user_id

英文

Refers to a SELECT statement that is embedded within an enclosing SELECT.

A subquery comes in two general flavors, one known as a “scalar select” which specifically must return exactly one row and one column, and the other form which acts as a “derived table” and serves as a source of rows for the FROM clause of another select. A scalar select is eligible to be placed in the WHERE clause, columns clause, ORDER BY clause or HAVING clause of the enclosing select, whereas the derived table form is eligible to be placed in the FROM clause of the enclosing SELECT.

Examples:

a scalar subquery placed in the columns clause of an enclosing
SELECT. The subquery in this example is a correlated subquery because part of the rows which it selects from are given via the enclosing statement.
SELECT id, (SELECT name FROM address WHERE address.user_id=user.id) FROM user
a scalar subquery placed in the WHERE clause of an enclosing
SELECT. This subquery in this example is not correlated as it selects a fixed result.
SELECT id, name FROM user WHERE status=(SELECT status_id FROM status_code WHERE code=’C’)
a derived table subquery placed in the FROM clause of an enclosing
SELECT. Such a subquery is almost always given an alias name.
SELECT user.id, user.name, ad_subq.email_address FROM

user JOIN (select user_id, email_address FROM address WHERE address_type=’Q’) AS ad_subq ON user.id = ad_subq.user_id

transient¶

瞬态¶

中文

这描述了对象在 Session 中可以具有的主要对象状态之一；一个瞬态对象是一个没有任何数据库标识的新对象，并且尚未与会话关联。当对象被添加到会话中时，它会移动到 pending 状态。

参见

英文

This describes one of the major object states which an object can have within a Session; a transient object is a new object that doesn’t have any database identity and has not been associated with a session yet. When the object is added to the session, it moves to the pending state.

参见