在使用alembic开发管理数据库时，会遇到一个比较麻烦的问题，就是变更某列的枚举类型，事实上使用sql命令变更相当的简单，一条alter的执行即可：

ALTER TYPE status ADD value 'output_limit_exceeded' after 'timed_out';
#删除
DROP TYPE status

但这样并不能满足alembic管理的初衷，也无法实现downgrade。

使用google搜索关键字“alembic enum type”，第一个出现的是stackflow的一个帖子Altering an Enum field using Alembic。

from alembic import opimport sqlalchemy as saold_options = ('nonexistent_executable', 'signal', 'success', 'timed_out')
new_options = sorted(old_options + ('output_limit_exceeded',))old_type = sa.Enum(*old_options, name='status')new_type = sa.Enum(*new_options, name='status')tmp_type = sa.Enum(*new_options, name='_status')tcr = sa.sql.table('testcaseresult',sa.Column('status', new_type, nullable=False))def upgrade():# Create a tempoary "_status" type, convert and drop the "old" typetmp_type.create(op.get_bind(), checkfirst=False)op.execute('ALTER TABLE testcaseresult ALTER COLUMN status TYPE _status'' USING status::text::_status')old_type.drop(op.get_bind(), checkfirst=False)# Create and convert to the "new" status typenew_type.create(op.get_bind(), checkfirst=False)op.execute('ALTER TABLE testcaseresult ALTER COLUMN status TYPE status'' USING status::text::status')tmp_type.drop(op.get_bind(), checkfirst=False)def downgrade():# Convert 'output_limit_exceeded' status into 'timed_out'op.execute(tcr.update().where(tcr.c.status==u'output_limit_exceeded').values(status='timed_out'))# Create a tempoary "_status" type, convert and drop the "new" typetmp_type.create(op.get_bind(), checkfirst=False)op.execute('ALTER TABLE testcaseresult ALTER COLUMN status TYPE _status'' USING status::text::_status')new_type.drop(op.get_bind(), checkfirst=False)# Create and convert to the "old" status typeold_type.create(op.get_bind(), checkfirst=False)op.execute('ALTER TABLE testcaseresult ALTER COLUMN status TYPE status'' USING status::text::status')tmp_type.drop(op.get_bind(), checkfirst=False)

这个方法提供了解决了问题，但稍显繁琐。我们可以做一下简单的封装，生成一个通用函数：

def upgrade_enum(table, column_name, enum_name, old_options, new_options):old_type = sa.Enum(*old_options, name=enum_name)new_type = sa.Enum(*new_options, name=enum_name)tmp_type = sa.Enum(*new_options, name="_" + enum_name)# Create a temporary type, convert and drop the "old" typetmp_type.create(op.get_bind(), checkfirst=False)op.execute(u'ALTER TABLE {0} ALTER COLUMN {1} TYPE _{2}'u' USING {1}::text::_{2}'.format(table,column_name,enum_name))old_type.drop(op.get_bind(), checkfirst=False)# Create and convert to the "new" typenew_type.create(op.get_bind(), checkfirst=False)op.execute(u'ALTER TABLE {0} ALTER COLUMN {1} TYPE {2}'u' USING {1}::text::{2}'.format(table,column_name,enum_name))tmp_type.drop(op.get_bind(), checkfirst=False)

这样就可以直接通过传参直接来执行升级或降级操作了。

操作环境：

 alembic-0.8.2
-bash-4.2$ psql --version
psql (PostgreSQL) 9.3.10