在本例中， user_to_dict() 函数实现了将 user 对象转换为 dict，但这种写法是硬编码的，有多少个 Model 就要写多少个函数，所以，我们接下来考虑如何实现通用的代码。

对于 User 的实例，可以用 __dict__ 特殊属性获取实例的字段，为了通用化，将代码放在一个基类中：

class EntityBase(object):
    def to_json(self):
        fields = self.__dict__
        if "_sa_instance_state" in fields:
            del fields["_sa_instance_state"]
        return fields
然后在 Model 中通过多重继承，让 model 继承 EntitiBase 类，这样就不用改变每个 model 类的代码。为了方便叙述和学习，也给出第二种方法的完整代码：
from flask import Flask, jsonify
from flask_sqlalchemy import SQLAlchemy
import os
from collections import OrderedDict
app = Flask(__name__)
# application configurations
app.config['SECURITY_KEY'] = 'you will never guess'
app.config['SQLALCHEMY_DATABASE_URI'] = os.environ['DB_URI']
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = True
app.config['JSON_SORT_KEYS'] = False
db = SQLAlchemy(app)
class EntityBase(object):
    def to_json(self):
        fields = self.__dict__
        if "_sa_instance_state" in fields:
            del fields["_sa_instance_state"]
        return fields
class User(db.Model, EntityBase):
    __tablename__ = 'user'
    USER_ID = db.Column(db.Integer, primary_key=True)
    USERNAME = db.Column(db.String(20))
    CREATED_BY = db.Column(db.String(20))
    CREATED_DATE = db.Column(db.Date)
@app.route('/users')
def list_users():
    users = User.query.all()
    users_output = []
    for user in users:
        users_output.append(user.to_json())
    return jsonify(users_output)
@app.route('/users/<userid>')
def find_user(userid):
    user = User.query.get(userid)
    return jsonify(user.to_json())
if __name__ == "__main__":
    app.run(debug=True)
如果有多个表，并且有关联，以上方法需要更多的手工代码。比较好的方法是使用第三方模块：Flask-Marshmallow，这个也是我想推荐的方法。通过 pip 安装 Flask-Marshmallow 模块后，我们需要定义一个 UserSchema 类，并且确定 UserSchema 对应到 User，如下面的代码：
from flask_marshmallow import Marshmallow
ma = Marshmallow(app)
class UserSchema(ma.ModelSchema):
    class Meta:
        model = User
然后在路由中，可以方便的通过 dump() 方法转换为 dict。
@app.route('/users')
def list_users():
    users = User.query.all()
    user_schema = UserSchema(many=True)
    user_data = user_schema.dump(users).data
    return jsonify(user_data)
@app.route('/users/<userid>')
def find_user(userid):
    user = User.query.get(userid)
    user_schema = UserSchema()
    user_data = user_schema.dump(user).data
    return jsonify(user_data)
完整代码如下：
from flask import Flask, jsonify
from flask_marshmallow import Marshmallow
from flask_sqlalchemy import SQLAlchemy
import os
app = Flask(__name__)
# application configurations
app.config['SECURITY_KEY'] = 'you will never guess'
app.config['SQLALCHEMY_DATABASE_URI'] = os.environ['DB_URI']
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = True
app.config['JSON_SORT_KEYS'] = False
db = SQLAlchemy(app)
ma = Marshmallow(app)
class User(db.Model):
    __tablename__ = 'user'
    USER_ID = db.Column(db.Integer, primary_key=True)
    USERNAME = db.Column(db.String(20), nullable=False)
    CREATED_BY = db.Column(db.String(20), nullable=False)
    CREATED_DATE = db.Column(db.Date, nullable=False)
class UserSchema(ma.ModelSchema):
    class Meta:
        model = User
@app.route('/users')
def list_users():
    users = User.query.all()
    user_schema = UserSchema(many=True)
    user_data = user_schema.dump(users).data
    return jsonify(user_data)
@app.route('/users/<userid>')
def find_user(userid):
    user = User.query.get(userid)
    user_schema = UserSchema()
    user_data = user_schema.dump(user).data
    return jsonify(user_data)
if __name__ == "__main__":
    app.run(debug=True)
源码放在 github 上。