Flask-SqlAlchemy composite key one to many relationship Sql Server - sql-server-2012

Question

I'm currently getting an error, I'm using sql server and trying to model a simple Parent with an array of Children:
sqlalchemy.exc.NoForeignKeysError: Could not determine join condition
between parent/child tables on relationship Parent.children- there are no
foreign keys linking these tables. Ensure that referencing columns
are associated with a ForeignKey or ForeignKeyConstraint, or specify a
'primaryjoin' expression.
my classes are set up simply as follows:
class Parent(db.Model):
__tablename__ = "parent"
parentId = db.Column(db.Integer, primary_key=True)
parentVersion = db.Column(db.Integer, primary_key=True)
children = db.relationship('Child', backref="parent",lazy=True)
class Child(db.Model):
__tablename__ = "child"
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(512), nullable=False)
parentId = db.Column(db.Integer, nullable=False)
parentVersion = db.Column(db.Integer, nullable=False)
ForeignKeyConstraint(['parentId', 'parentVersion'], ['parent.parentId', 'parent.parentVersion']
I've tried fiddling with declaring the relationship and foreign key in several ways but i always get an error, what is the correct way to do this?

Answer 1

Your forgot to add a foreign key:
parentId = db.Column(db.Integer, db.ForeignKey("parent.id))
There is a lot of documentation material regarding this topic, if there is still anything unclear to you.

Answer 2

You are missing adding the ForeignKeyConstraint to the table args, and you are using camel case, not snake case. And you don't need the __tablename__ with Flask-SQLAlchemy.
Try:
class Parent(db.Model):
id = db.Column(db.Integer, primary_key=True)
version = db.Column(db.Integer, primary_key=True)
children = db.relationship('Child', backref="parent", lazy=True)
class Child(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(512), nullable=False)
parent_id = db.Column(db.Integer, nullable=False)
parent_version = db.Column(db.Integer, nullable=False)
__table_args__ = (
db.ForeignKeyConstraint(
['parent_id', 'parent_version'],
['parent.id', 'parent.version']
),
)