Flask-SqlAlchemy允许重复的多对多关系

Question

我有一个Order - FoodItem多对多的关系,如下所示:

association_table = db.Table('association', db.Model.metadata,
    db.Column('left_id', db.Integer, db.ForeignKey('orders.order_id')),
    db.Column('right_id', db.Integer, db.ForeignKey('fooditems.fooditem_id'))
)

class OrderModel(ReviewableModel):
    __tablename__ = 'orders'
    order_id = db.Column(db.Integer, db.ForeignKey('reviewables.id'), primary_key=True)
    food_items = db.relationship("FoodItemModel", secondary = association_table)
    __mapper_args__ = {'polymorphic_identity':'orders'}

class FoodItemModel(ReviewableModel):
    __tablename__ = 'fooditems'
    fooditem_id = db.Column(db.Integer, db.ForeignKey('reviewables.id'), primary_key=True)  
    __mapper_args__ = {'polymorphic_identity':'fooditems'}

用户可以请求具有重复foodItem的订单.这是正确创建的,但是当我将更改保存到数据库时,将删除重复项.例如,我订购3个比萨饼:

def save_to_db(self):
    print('before: '+str(self.food_items))
    db.session.add(self)
    db.session.commit()
    print('after: '+str(self.food_items))

输出是这样的:

before: [<FoodItemModel u'Pizza'>, <FoodItemModel u'Pizza'>, <FoodItemModel u'Pizza'>]
after: [<FoodItemModel u'Pizza'>]

关联表已正确更新:

"left_id"   "right_id"
"6"         "3"
"6"         "3"
"6"         "3"

但是,OrderModel中的food_items只包含1个项目

Answer 1

Juan Mellado在他的回答中得到的是关系数据(RD)和对象关系映射(ORM)冲突:ORM无法区分具有相同数据的单独对象.为了解决这个问题,只需添加一个id作为主键列association_table-这样的ORM有什么用相同的区分不同的记录left_id和right_id.

但这将是一种解决方法,而不是解决方案.

解决方案是考虑"用户可以请求具有重复foodItems的订单"时的含义.从订单到食品的关系不是直接的,而是通过订单项间接的.每个订单商品属于订单(其又属于顾客或餐桌),并且每个订单商品可以与食品相关.通过使每个订单项唯一,"重复食品"的问题消失.同时,我们现在可以通过向每个订单商品添加可选的"客户请求"来获得无限量的食品变化.例如"食物:薯条,要求:容易上盐".

在代码中的示例下,客户"我尖叫"放置1份订单,其中3份"冰淇淋",​​其中1份"洒在上面".

from sqlalchemy import Column, Integer, String, ForeignKey, create_engine
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.pool import StaticPool

Base = declarative_base()

class Order(Base):
    __tablename__ = 'order'
    id = Column(Integer, primary_key=True)
    customer = Column(String(127))
    items = relationship("OrderItem")
    def __repr__(self):
        return "<Order(id='{}', customer='{}', items='{}')>".format(self.id, self.customer, self.items)

class Food(Base):
    __tablename__ = 'food'
    id = Column(Integer, primary_key=True)
    name = Column(String(127))
    def __repr__(self):
        return "<Food(id='{}', name='{}')>".format(self.id, self.name)

class OrderItem(Base):
    __tablename__ = 'order_item'
    id = Column(Integer, primary_key=True)
    order_id = Column(Integer, ForeignKey(Order.id))
    order = relationship(Order)
    food_id = Column(Integer, ForeignKey(Food.id))
    food = relationship(Food)
    comment = Column(String(127))
    def __repr__(self):
        return "<OrderItem(id='{}', order_id='{}', food_id='{}, comment={}')>" \
            .format(self.id, self.order_id, self.food_id, self.comment)

def orderFood():
    engine = create_engine('sqlite:///:memory:', echo=True, connect_args={'check_same_thread':False}, poolclass=StaticPool)
    Base.metadata.create_all(engine)
    Session = sessionmaker(bind=engine)
    session = Session()
    food = Food(name='ice cream')
    session.add(food)
    order = Order(customer='I scream')
    session.add(order)
    session.commit()
    print("Food: {}".format(food))
    print("Order: {}".format(order))
    order.items = [OrderItem(order=order, food=food), OrderItem(order=order, food=food), \
                   OrderItem(order=order, food=food, comment='with sprinkles on top')]
    session.merge(order)
    session.commit()
    print("Order: {}".format(order))
    print("Order.items")
    for item in order.items:
        print(item)
    print("OrderItems for order")
    orderFilter = OrderItem.order_id == order.id
    for order_item in session.query(OrderItem).filter(orderFilter).all():
        print(order_item)
    print("Food in order")
    for row in session.query(Food).join(OrderItem).filter(orderFilter).all():
        print(row)
    session.close();

if __name__ == "__main__":
    orderFood()

Answer 2

您必须为关联表声明主键。

Flask-SQLALchemy 是一个 ORM，它需要一系列唯一标识一行的列。

看一下文档的这一部分，有点过时，但仍然有效： http ://docs.sqlalchemy.org/en/rel_1_1/faq/ormconfiguration.html#faq-mapper-primary-key

Flask-SQLALchemy 使用所有字段（left_id、right_id）来标识行，并且所有行都具有相同的值（6、3）。因此，所有行都存储在数据库中（因为没有任何声明的约束），但只有一行保留在上下文（内存）中。