Flask Marshmallow / SqlAlchemy：序列化多对多关系

Question

我正在使用Flask，flask-sqlalchemy和flask-marshmallow构建一个小型REST api。 对于某些请求，我想返回一个由我的sqlalchemy对象组成的json序列化响应。 但是，当使用多对多关系/辅助表时，我无法使用急切加载的sqlalchemy对象进行序列化。

这是一个简单的例子，或多或少地从flask-marshmallow docs复制/粘贴：

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_marshmallow import Marshmallow
from sqlalchemy.orm import joinedload

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite://'

# Order matters: Initialize SQLAlchemy before Marshmallow
db = SQLAlchemy(app)
ma = Marshmallow(app)

secondary_foo = db.Table('secondary_foo',
                            db.Column('author_id', db.Integer, db.ForeignKey('author.id')),
                            db.Column('book_id', db.Integer, db.ForeignKey('book.id')))

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(255))

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(255))
    authors = db.relationship('Author', secondary="secondary_foo", backref='books', lazy="joined")

class AuthorSchema(ma.ModelSchema):
    class Meta:
        model = Author

class BookSchema(ma.ModelSchema):
    #authors = ma.Nested(AuthorSchema) <-- Doesn't work, authors will be serialized to empty json object, instead of list of ids
    class Meta:
        model = Book


db.drop_all()
db.create_all()
author_schema = AuthorSchema()
book_schema = BookSchema()
author = Author(name='Chuck Paluhniuk')
book = Book(title='Fight Club')
book.authors.append(author)
db.session.add(author)
db.session.add(book)
db.session.commit()

s = BookSchema(many=True)

基于上面的代码，我可以热切地加载书籍并获取作者对象。 但是，当序列化深层对象被序列化为ID列表时：

print(Book.query.filter(1==1).options(joinedload('authors')).all()[0].authors)
//--> [<__main__.Author object at 0x1043a0dd8>]

print(s.dump(Book.query.filter(1==1).options(joinedload('authors')).all()).data)
//--> [{'authors': [1], 'title': 'Fight Club', 'id': 1}]

这是我想要的结果：

print(s.dump(Book.query.filter(1==1).options(joinedload('authors')).all()).data)
//--> [{'authors': [{'name':'Chuck Paluhniuk', 'id':'1'}], 'title': 'Fight Club', 'id': 1}]

我怎么做？

Answer 1

在BookSchema ，您将要添加一个Nested authors字段（但已注释掉），但您需要使用many关键字参数指定它是一个列表。

class BookSchema(ma.ModelSchema):

    # A list of author objects
    authors = ma.Nested(AuthorSchema, many=True)

    class Meta:
        model = Book