flask-migrate 无法删除表，因为其他对象依赖于它答案

【问题标题】：flask-migrate cannot drop table because other objects depend on itflask-migrate 无法删除表，因为其他对象依赖于它
【发布时间】：2018-09-01 00:05:15
【问题描述】：

通过 Flask-Migrate (SQLAlchemy) 为 PostgresSQL DB 应用刚刚创建的迁移（添加的 db.Model）时出现问题。错误本身：

sqlalchemy.exc.InternalError: (psycopg2.InternalError) cannot drop table parameter_subtype because other objects depend on it

完整的错误堆栈跟踪是：

INFO  [alembic.autogenerate.compare] Detected removed foreign key (event_id) (id) on table stage_has_event
INFO  [alembic.autogenerate.compare] Detected removed column 'stage_has_event.event_id'
  Generating /app/migrations/versions/e224df1a4818_.py ... done
(venv) $ ./manage db upgrade
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade cadf22871ae0 -> e224df1a4818, empty message
Traceback (most recent call last):
  File "/app/venv/bin/flask", line 11, in <module>
    sys.exit(main())
  File "/app/venv/lib/python2.7/site-packages/flask_cli/cli.py", line 502, in main
    cli.main(args=args, prog_name=name)
  File "/app/venv/lib/python2.7/site-packages/flask_cli/cli.py", line 369, in main
    return AppGroup.main(self, *args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/flask/cli.py", line 257, in decorator
    return __ctx.invoke(f, *args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/app/venv/lib/python2.7/site-packages/flask_migrate/cli.py", line 134, in upgrade
    _upgrade(directory, revision, sql, tag, x_arg)
  File "/app/venv/lib/python2.7/site-packages/flask_migrate/__init__.py", line 259, in upgrade
    command.upgrade(config, revision, sql=sql, tag=tag)
  File "/app/venv/lib/python2.7/site-packages/alembic/command.py", line 254, in upgrade
    script.run_env()
  File "/app/venv/lib/python2.7/site-packages/alembic/script/base.py", line 427, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/app/venv/lib/python2.7/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
    module = load_module_py(module_id, path)
  File "/app/venv/lib/python2.7/site-packages/alembic/util/compat.py", line 141, in load_module_py
    mod = imp.load_source(module_id, path, fp)
  File "migrations/env.py", line 87, in <module>
    run_migrations_online()
  File "migrations/env.py", line 80, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/app/venv/lib/python2.7/site-packages/alembic/runtime/environment.py", line 836, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/app/venv/lib/python2.7/site-packages/alembic/runtime/migration.py", line 330, in run_migrations
    step.migration_fn(**kw)
  File "/app/migrations/versions/e224df1a4818_.py", line 21, in upgrade
    op.drop_table('parameter_subtype')
  File "<string>", line 8, in drop_table
  File "<string>", line 3, in drop_table
  File "/app/venv/lib/python2.7/site-packages/alembic/operations/ops.py", line 1187, in drop_table
    operations.invoke(op)
  File "/app/venv/lib/python2.7/site-packages/alembic/operations/base.py", line 319, in invoke
    return fn(self, operation)
  File "/app/venv/lib/python2.7/site-packages/alembic/operations/toimpl.py", line 70, in drop_table
    operation.to_table(operations.migration_context)
  File "/app/venv/lib/python2.7/site-packages/alembic/ddl/impl.py", line 203, in drop_table
    self._exec(schema.DropTable(table))
  File "/app/venv/lib/python2.7/site-packages/alembic/ddl/impl.py", line 118, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1009, in _execute_ddl
    compiled
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1200, in _execute_context
    context)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1413, in _handle_dbapi_exception
    exc_info
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/app/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1193, in _execute_context
    context)
  File "/app`enter code here`/venv/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 507, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.InternalError: (psycopg2.InternalError) cannot drop table parameter_subtype because other objects depend on it
DETAIL:  constraint parameter_parameter_subtype_id_fkey on table parameter depends on table parameter_subtype
HINT:  Use DROP ... CASCADE to drop the dependent objects too.
 [SQL: '\nDROP TABLE parameter_subtype']

这导致表也没有在 PostgreSQL 数据库和列中创建。不确定是什么原因导致了此问题以及如何理解此错误消息。

我确实手动删除了表格，它有所帮助。但是如何与迁移。有什么建议吗？

【问题讨论】：

HINT: Use DROP ... CASCADE to drop the dependent objects too. 应该是一个提示。您必须实现处理这些依赖对象的迁移逻辑。一个简单的方法（但可能不适合您的使用！）是 CASCADE，它也会删除那些相关的对象。您希望通过运行迁移来做到这一点，以确保您能够安全地回滚并处理迁移错误。在迁移文件不知情的情况下手动删除内容会带来一些风险。
我确实通过本地数据库上的 pgAdmin 删除了它们。一切似乎都在工作。但是我似乎没有在其他模型中找到任何参考。好像我都看过了。
在迁移中做 DROP ... CASCADE 的任何方法？
就像我说的，在迁移文件不知道的情况下这样做（从 pgAdmin 删除）是有风险的。换句话说，您的迁移预期某个状态不再与数据库的实际状态一致。如果您发布您的模型并描述您在 pgAdmin 中采取的确切步骤，这可能会有所帮助。
谢谢。至少我知道在哪里看...

标签： python postgresql sqlalchemy alembic

【解决方案1】：

Flask-Migrate 不会读取数据库以查看对象之间的依赖关系。您可以通过重新排序迁移文件中的 drop_table 来实现成功迁移。

您可以像这样修改迁移文件：

op.drop_table("parameter_subtype")
op.drop_table("parameter")

然后运行升级命令。通常按此顺序订购 drop 应该可以解决问题。

【讨论】：

“op”从何而来？
@FrederikChristoffersen from alembic import op

【解决方案2】：

我遇到了同样的错误，对我有用的是首先使用以下命令将表删除到数据库中：

$ DROP TABLE table_name;

然后在升级（）函数中的迁移文件中删除或注释掉它说的行：

op.drop_table('table_name')

【讨论】：