通过Flask-Migrate（SQLAlchemy）为PostgresQL DB应用刚刚创建的迁移（添加了db.Model）的问题。错误本身：
sqlalchemy.exc.InternalError: (psycopg2.InternalError) cannot drop table parameter_subtype because other objects depend on it
完整错误堆栈跟踪是：
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']
这导致表没有在PostgreSQL数据库和列中创建。不确定可能导致此问题的原因以及如何理解此错误消息。
我做了手动删除表，这有帮助。但是如何进行迁移。有什么建议？
Flask-Migrate不会在数据库中读取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。
您可以做的是修改迁移文件，如下所示：
op.drop_table("parameter_subtype")
op.drop_table("parameter")
然后运行upgrade命令。通常按此顺序排序掉落应该可以解决问题。