Flask-SQLAlchemy how to store list of ForeignKey Objects

Question

I am creating an application that will gather requirements for a number of different 'Types' of items, and can't figure out how to structure the Flask-SQLAlchemy ORM Model relationship(s).

I have the following Classes:

ItemSize - A 'Type', think of liek a T-Shirt size.

class ItemSize(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True, nullable=False)
    date_created = db.Column(db.Date(), default=datetime.now())
    height = db.Column(db.Integer, nullable=True)
    depth = db.Column(db.Integer, nullable=True)
    width = db.Column(db.Integer, nullable=True)

ItemSet - A collection of a particular ItemSize, stores relationship to ItemSize and a Count. (Eg. ItemSize: 'A', Count: 2)

class ItemSet(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    size = db.Column(db.Integer, db.ForeignKey('itemsizes.id'), nullable=True)
    count = db.Column(db.Integer, nullable=False)

Requirements - A collection of many ItemSets. (Eg. ItemSets [1, 2, 3, 4, 5])

class Specification(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True, nullable=False)
    comments = db.Column(db.String(255), nullable=True)
    
    # THIS IS THE BIT I AM UNSURE OF
    # Need this to a collection of Many ItemSets
    # Will the below work? 
    requirements = db.relationship('ItemSet', uselist=True, backref='itemsets')    

The above gives me the following error when trying to create any objects:

sqlalchemy.exc.InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers. Triggering mapper: 'mapped class Specification->specifications'. Original exception was: Could not determine join condition between parent/child tables on relationship Specification.requirements - there are no foreign keys linking these tables.  Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression.

Does anyone know how to achieve this kind of relationship?

Specification.requirements -> [Array of ItemSets]

Any pointers very much appreciated.

Answer 1

You need to modify these 2 classes:

class Specification(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True, nullable=False)
    comments = db.Column(db.String(255), nullable=True)
    
    # THIS IS THE BIT I AM UNSURE OF
    # Need this to a collection of Many ItemSets
    # Will the below work? 
    requirements = relationship("ItemSet", back_populates="spec")

class ItemSet(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    size = db.Column(db.Integer, db.ForeignKey('itemsizes.id'), nullable=True)
    count = db.Column(db.Integer, nullable=False)
    spec_id = Column(db.Integer, ForeignKey('spec.id'))
    spec = relationship("Specification", back_populates="requirements")