Querying with function on Flask-SQLAlchemy model gives BaseQuery object is not callable error

Model.query is a shortcut to db.session.query(Model), it's not callable. If you're not querying a model, continue to use db.session.query(...) as you would with regular SQLAlchemy.

db.session.query(db.func.sum(Services.price)).filter(
    Services.dateAdd.between(start, end)
)