Question

I have an SQLAlchemy model like the one below, and at first it didn’t work (problems with the join, and then expecting a scalar instead of a list). I “fixed” it in the included version, but I really can’t understand why it behaved like that.

At first I expected that with those ForeignKeys the Sizes.items relationship() shouldn’t need an explicit primaryjoin, and when I added it SA started expecting a scalar and I had to explicitly specify uselist=True.

Why doesn’t the relationship automatically detect one or both those things?

class Category(Base):
    __tablename__ = 'categories'
    pk   = Column(String(6), primary_key=True)

class Item(Base):
    __tablename__ = 'items'
    pk          = Column(String(6), primary_key=True)
    category_pk = Column(String(6), ForeignKey('categories.pk') )
    size        = Column(Integer(), nullable=False)
    category    = relationship('Category', backref=backref('items'))

class Sizes(Base):
    __tablename__ = 'sizes'
    category_pk = Column(String(6), ForeignKey('categories.pk'),
                    ForeignKey('items.category_pk'), primary_key=True )
    size        = Column(Integer(), ForeignKey('items.size'), primary_key=True )
    category    = relationship('Category', backref=backref('sizes'))
    items       = relationship('Item',
        uselist=True,
        primaryjoin="and_(Sizes.category_pk==Item.category_pk, Sizes.size==Item.size)" )

Answer 1

I believe what is happening is that you have two foreign keys, not a single FK on two columns. It works a bit different the primary_key=True where you can do that.

Try something like:

class Category(Base):
    __tablename__ = 'categories'
    pk   = Column(String(6), primary_key=True)

class Item(Base):
    __tablename__ = 'items'
    pk          = Column(String(6), primary_key=True)
    category_pk = Column(String(6), ForeignKey('categories.pk') )
    size        = Column(Integer(), nullable=False)
    category    = relationship('Category', backref=backref('items'))

class Sizes(Base):
    __tablename__ = 'sizes'
    category_pk = Column(String(6), primary_key=True )
    size        = Column(Integer(), primary_key=True )
    category    = relationship('Category', backref=backref('sizes'))
    items       = relationship('Item')

    __table_args__ = (
        ForeignKeyConstraint(
            ["category_pk", "size"],
            ["items.category_pk", "items.size"]
        ),
    )