在sqlalchemy中复合两个外键？

Question

I am using SQLALchemy in flask. And I am confused how to define composite key with two foreign keys. One foreign key from one table and second from another table.

class Store_category_mapping(db.Model):

    __tablename__ = 'store_category_mapping' 
    category_id = db.Column(db.Integer, nullable=False) 
    store_id = db.Column(db.Integer, nullable=False)
    store_name    = db.Column(db.String(50),  nullable=False)

    __table_args__ = (
        db.ForeignKeyConstraint(
            [category_id, store_id],
            [Category_master.category_id, Store_master.store_id]
        ),
        )

It gives this error:

sqlalchemy.exc.ArgumentError: ForeignKeyConstraint on store_category_mapping(category_id, store_id) refers to multiple remote tables: category_master and store_master

Answer 1

You can't have a composite foreign key that references more than one remote table. Composite foreign keys are a way to ensure integrity when one remote table has a composite primary key.

If you want to make sure that you only ever have one store_category_mapping entry with a given combination of category_id and store_id you could go for a UniqueConstraint instead.

I'm not familiar with flask, but I assume it would look somewhat like this:

class Store_category_mapping(db.Model):

    __tablename__ = 'store_category_mapping' 
    category_id = db.Column(db.Integer, db.ForeignKey(Category_master.category_id), nullable=False) 
    store_id = db.Column(db.Integer, db.ForeignKey(Store_master.store_id), nullable=False)
    store_name    = db.Column(db.String(50),  nullable=False)

    __table_args__ = (
        db.UniqueConstraint('category_id','store_id'),
        )