SQLAlchemy - How can I make eager loading count property

Question

I want to make a property for model which contains count.

Since I always need the property, I want to make query with JOIN like sqlalchemy.orm.relatio

Answer 1

I solved it by using sqlalchemy.orm.column_property

I replaced the foo_count by following

import sqlalchemy as s, func, select
from sqlalchemy.orm import relationship, column_property

# ...

class Foo(Base):
    __tablename__ = 'foo'
    id = s.Column(s.Integer, primary_key=True)
    bar_id = s.Column(s.Integer, s.ForeignKey('bar.id'))
    bar = relationship('Bar')


class Bar(Base):
    __tablename__ = 'bar'
    id = s.Column(s.Integer, primary_key=True)

    foo_count = column_property(
        select([func.count(Foo.id)])
        .where(Foo.bar_id == id)
    )