Referring to all heads at once

一次引用到所有的head

The heads identifier is a lot like head, except it explicitly refers to all heads at once. That is, it’s like telling Alembic to do the operation for both ae1027a6acf and 27c6a30d7c24 simultaneously. If we started from a fresh database and ran upgrade heads we’d see:

heads 标识符很像 head，除了它一次明确地引用所有 heads。也就是说，这就像告诉 Alembic 对 ae1027a6acf 和 27c6a30d7c24 同时进行操作。如果我们从一个新的数据库开始并运行 upgrade heads，我们会看到：

$ alembic upgrade heads
INFO  [alembic.migration] Context impl PostgresqlImpl.
INFO  [alembic.migration] Will assume transactional DDL.
INFO  [alembic.migration] Running upgrade  -> 1975ea83b712, create account table
INFO  [alembic.migration] Running upgrade 1975ea83b712 -> ae1027a6acf, add a column
INFO  [alembic.migration] Running upgrade 1975ea83b712 -> 27c6a30d7c24, add shopping cart table

Since we’ve upgraded to heads, and we do in fact have more than one head, that means these two distinct heads are now in our alembic_version table. We can see this if we run alembic current:

由于我们已经升级到 heads，而且我们实际上有不止一个 head ，这意味着这两个不同的 heads 现在在我们的 alembic_version 表中。 如果我们运行alembic current，我们可以看到这一点：

$ alembic current
ae1027a6acf (head)
27c6a30d7c24 (head)

That means there’s two rows in alembic_version right now. If we downgrade one step at a time, Alembic will delete from the alembic_version table each branch that’s closed out, until only one branch remains; then it will continue updating the single value down to the previous versions:

这意味着现在 alembic_version 中有两行。 如果我们一次降级一个步骤，Alembic 将从 alembic_version 表中 删除 每个关闭的分支，直到只剩下一个分支； 然后它将继续将单个值更新到以前的版本：

$ alembic downgrade -1
INFO  [alembic.migration] Running downgrade ae1027a6acf -> 1975ea83b712, add a column

$ alembic current
27c6a30d7c24 (head)

$ alembic downgrade -1
INFO  [alembic.migration] Running downgrade 27c6a30d7c24 -> 1975ea83b712, add shopping cart table

$ alembic current
1975ea83b712 (branchpoint)

$ alembic downgrade -1
INFO  [alembic.migration] Running downgrade 1975ea83b712 -> , create account table

$ alembic current