在flask项目中，为了避免循环另外建了个py存放db = SQLAlchemy()，然后在app中引用，然后，问题出现了！
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()
from flask import Flask, request, render_template, session, redirect
import config
from flask_migrate import Migrate
from datetime import datetime
from models import SubmissionModel
from middle import db
app = Flask(__name__)
app.config.from_object(config)
db.init_app(app)
migrate = Migrate(app, db)
File "E:\birdfoly.com\flaskProject\models.py", line 7, in
from middle import db
File "E:\birdfoly.com\flaskProject\middle.py", line 8, in
db = SQLAlchemy()
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
求解救！
发生异常: TypeError
metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
File "E:\birdfoly.com\flaskProject\middle.py", line 8, in
db = SQLAlchemy()
File "E:\birdfoly.com\flaskProject\models.py", line 7, in
from middle import db
File "E:\birdfoly.com\flaskProject\app.py", line 4, in
from models import SubmissionModel
