Conditional Migration Elements

条件迁移元素

This example features the basic idea of a common need, that of affecting how a migration runs based on command line switches.

此示例具有共同需求的基本思想，即基于命令行开关影响迁移的运行方式。

The technique to use here is simple; within a migration script, inspect the EnvironmentContext.get_x_argument() collection for any additional, user-defined parameters. Then take action based on the presence of those arguments.

这里使用的技术很简单；在迁移脚本中，检查 EnvironmentContext.get_x_argument() 集合中是否有任何其他用户定义的参数。然后根据这些参数的存在采取行动。

To make it such that the logic to inspect these flags is easy to use and modify, we modify our script.py.mako template to make this feature available in all new revision files:

为了使检查这些标志的逻辑易于使用和修改，我们修改了我们的 script.py.mako 模板，以使该功能在所有新修订文件中都可用：

"""${message}

Revision ID: ${up_revision}
Revises: ${down_revision}
Create Date: ${create_date}

"""

# revision identifiers, used by Alembic.
revision = ${repr(up_revision)}
down_revision = ${repr(down_revision)}

from alembic import op
import sqlalchemy as sa
${imports if imports else ""}

from alembic import context


def upgrade():
    schema_upgrades()
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_upgrades()

def downgrade():
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_downgrades()
    schema_downgrades()

def schema_upgrades():
    """schema upgrade migrations go here."""
    ${upgrades if upgrades else "pass"}

def schema_downgrades():
    """schema downgrade migrations go here."""
    ${downgrades if downgrades else "pass"}

def data_upgrades():
    """Add any optional data upgrade migrations here!"""
    pass

def data_downgrades():
    """Add any optional data downgrade migrations here!"""
    pass

Now, when we create a new migration file, the data_upgrades() and data_downgrades() placeholders will be available, where we can add optional data migrations:

现在，当我们创建一个新的迁移文件时， data_upgrades() 和 data_downgrades() 新增的函数将可用，我们可以在其中添加可选的数据迁移：

"""rev one

Revision ID: 3ba2b522d10d
Revises: None
Create Date: 2014-03-04 18:05:36.992867

"""

# revision identifiers, used by Alembic.
revision = '3ba2b522d10d'
down_revision = None

from alembic import op
import sqlalchemy as sa
from sqlalchemy import String, Column
from sqlalchemy.sql import table, column

from alembic import context

def upgrade():
    schema_upgrades()
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_upgrades()

def downgrade():
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_downgrades()
    schema_downgrades()

def schema_upgrades():
    """schema upgrade migrations go here."""
    op.create_table("my_table", Column('data', String))

def schema_downgrades():
    """schema downgrade migrations go here."""
    op.drop_table("my_table")

def data_upgrades():
    """Add any optional data upgrade migrations here!"""

    my_table = table('my_table',
        column('data', String),
    )

    op.bulk_insert(my_table,
        [
            {'data': 'data 1'},
            {'data': 'data 2'},
            {'data': 'data 3'},
        ]
    )

def data_downgrades():
    """Add any optional data downgrade migrations here!"""

    op.execute("delete from my_table")

To invoke our migrations with data included, we use the -x flag:

要调用包含数据的迁移，我们使用 -x 标志：

alembic -x data=true upgrade head

The EnvironmentContext.get_x_argument() is an easy way to support new commandline options within environment and migration scripts.

EnvironmentContext.get_x_argument() 是一种在环境和迁移脚本中支持新命令行选项的简单方法。

alembic 中文文档

Conditional Migration Elements