python之'Marshmallow' 对象没有属性 'ModelSchema'

文档中的一切看起来都很好，但是当我运行应用程序时它仍然给我这个错误:

  File "main.py", line 21, in <module> 
    class UserSchema(ma.ModelSchema): 
AttributeError: 'Marshmallow' object has no attribute 'ModelSchema' 

一切都正确导入。数据库已提交。该行为在 pipenv 和 venv 上是相同的。

我错过了什么吗？

from flask import Flask, jsonify 
from flask_sqlalchemy import SQLAlchemy  
from flask_marshmallow import Marshmallow 
 
app = Flask(__name__) 
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///marshmallowjson.db' 
 
db  = SQLAlchemy(app) 
ma = Marshmallow(app) 
 
class User(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    name = db.Column(db.String(50)) 
 
class Item(db.Model): 
    id = db.Column(db.Integer, primary_key=True) 
    item_name = db.Column(db.String(50)) 
    user_id = db.Column(db.Integer, db.ForeignKey('user.id')) 
    user = db.relationship('User', backref='items') 
 
class UserSchema(ma.ModelSchema): 
    class Meta: 
        model = User  
         
class ItemSchema(ma.ModelSchema): 
    class Meta: 
        model = Item 
 
@app.route('/') 
def index(): 
    users = User.query.all() 
    user_schema = UserSchema(many=True) 
    output = user_schema.dump(users).data 
    return jsonify({'user': output}) 
 
if __name__ == '__main__': 
    app.run(debug=True) 

请您参考如下方法:

Conf.py

from flask_sqlalchemy import SQLAlchemy 
from flask_marshmallow import Marshmallow 
 
db = SQLAlchemy(app) 
ma = Marshmallow(app)  

# flask-marshmallow<0.12.0

class UserSchema(ma.ModelSchema): 
      class Meta: 
            model = User 

# flask-marshmallow>=0.12.0(推荐)

from conf import ma 
class UserSchema(ma.SQLAlchemyAutoSchema): 
      class Meta: 
            model = User 
            load_instance = True 

# flask-marshmallow>=0.12.0(不推荐)

from marshmallow_sqlalchemy import ModelSchema 
class UserSchema(ModelSchema): 
      class Meta: 
            model = User 
            sql_session = db.session