Generated column (row_number) in Flask SQL-Alchemy - sqlalchemy

Question

I'm trying to have a additional property on my object-relationship-mapped object.
I need to have a property that is calculated at the database using the ROW_NUMBER() function.
My Class looks something like this:
class Example(db.Model):
id = db.Column(db.Integer, primary_key=True)
group = db.Column(db.Integer)
I think I need to use the hybrid_property for this. I don't know to define the property but I think I have an idea about the expression:
class Example(db.Model):
id = db.Column(db.Integer, primary_key=True)
group_id = db.Column(db.Integer)
#hybrid_property
def group_specific_id(self):
# no idea what to put here
pass
#group_specific_id.expression
def project_specific_id(cls):
return db.session.query(func.row_number().over(partition_by=cls.group_id))
Am I on the right path? Can anyone help me here?

Answer 1

This definition should work for you:
class Example(db.Model):
id = db.Column(db.Integer, primary_key=True)
group_id = db.Column(db.Integer)
#hybrid_property
def group_specific_id(self):
return db.session.query(func.row_number().over(partition_by=self.group_id))
#group_specific_id.expression
def group_specific_id(cls):
return db.session.query(func.row_number().over(partition_by=cls.group_id))
And now you can use hybrid property like that:
example = Example(group_id=2)
example.group_specific_id
or like an expression
examples = session.query(Example).filter(Example.group_specific_id == <smth>)