Don’t emit DROP INDEX when the table is to be dropped as well

MySQL may complain when dropping an index that is against a column that also has a foreign key constraint on it. If the table is to be dropped in any case, the DROP INDEX isn’t necessary. This recipe will process the set of autogenerate directives such that all DropIndexOp directives are removed against tables that themselves are to be dropped:

MySQL 可能会在删除针对列的索引时抱怨，该列也具有外键约束。 如果要在任何情况下删除表，则不需要 DROP INDEX。 此配方将处理自动生成指令集，以便针对本身要删除的表删除所有 DropIndexOp 指令：

def run_migrations_online():

    # ...

    from alembic.operations import ops

    def process_revision_directives(context, revision, directives):
        script = directives[0]

        # process both "def upgrade()", "def downgrade()"
        for directive in (script.upgrade_ops, script.downgrade_ops):

            # make a set of tables that are being dropped within
            # the migration function
            tables_dropped = set()
            for op in directive.ops:
                if isinstance(op, ops.DropTableOp):
                    tables_dropped.add((op.table_name, op.schema))

            # now rewrite the list of "ops" such that DropIndexOp
            # is removed for those tables.   Needs a recursive function.
            directive.ops = list(
                _filter_drop_indexes(directive.ops, tables_dropped)
            )

    def _filter_drop_indexes(directives, tables_dropped):
        # given a set of (tablename, schemaname) to be dropped, filter
        # out DropIndexOp from the list of directives and yield the result.

        for directive in directives:
            # ModifyTableOps is a container of ALTER TABLE types of
            # commands.  process those in place recursively.
            if isinstance(directive, ops.ModifyTableOps) and \
                    (directive.table_name, directive.schema) in tables_dropped:
                directive.ops = list(
                    _filter_drop_indexes(directive.ops, tables_dropped)
                )

                # if we emptied out the directives, then skip the
                # container altogether.
                if not directive.ops:
                    continue
            elif isinstance(directive, ops.DropIndexOp) and \
                    (directive.table_name, directive.schema) in tables_dropped:
                # we found a target DropIndexOp.   keep looping
                continue

            # otherwise if not filtered, yield out the directive
            yield directive

    # connectable = ...

    with connectable.connect() as connection:
        context.configure(
            connection=connection,
            target_metadata=target_metadata,
            process_revision_directives=process_revision_directives
        )

        with context.begin_transaction():
            context.run_migrations()

Whereas autogenerate, when dropping two tables with a foreign key and an index, would previously generate something like:

在自动生成的情况下，当删除具有外键和索引的两个表时，以前会生成如下内容：

def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_index(op.f('ix_b_aid'), table_name='b')
    op.drop_table('b')
    op.drop_table('a')
    # ### end Alembic commands ###

With the above rewriter, it generates as:

使用上面的重写后，它生成为：

def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_table('b')
    op.drop_table('a')
    # ### end Alembic commands ###