Count related and filter in SQLAlchemy - flask

Question

So I have an object Services, object Services has a foreign key to object Waiters. Object waiter has also got a time field "exit" which must be smaller than the current time. The idea is to return all objects Waiter with a count for how many objects service they are related to that have a field exit larger than the current time. Then I have to do the same with Tables.
Finally I need to filter out the Waiters which currently hold four or more current services, and I have to filter out the tables with one or more.
Here are my models:
class Waiter(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(128))
service = db.relationship('Service', backref='waiter', lazy=True)
class Table(db.Model):
id = db.Column(db.Integer, primary_key=True)
table_no = db.Column(db.String(128))
service = db.relationship('Service', backref='table', lazy=True)
class Service(db.Model):
id = db.Column(db.Integer, primary_key=True)
table = db.Column(db.Integer, db.ForeignKey('table.id'))
waiter = db.Column(db.Integer, db.ForeignKey('waiter.id'))
arrival = db.Column(db.DateTime, default=datetime.datetime.now(mx))
exit = db.Column(db.DateTime)
tip = db.Column(db.Numeric(10,2))
So far I've got something like:
waiters = Waiter.query(Waiter.id, \
func.count(*"I don't know what to put here"*).\
label("service_count")).groupby(Waiter.id).subquery()
EDIT:
After further research I have come to the following query. I still don't know if it's right:
waiters = Waiter.query(Waiter.id, \
func.count("*").filter(Service.exit>=datetime.datetime.now()) .\
label("service_count")).groupby(Waiter.id).subquery()
Any help is much appreciated. Thank you very much.

Answer 1

I got an answer that does what I want but instead of of an annotation I get a tupple with the values. It's this:
waiters = db.session.query(Waiter, func.count(Service.id).\
filter(Service.exit>time)).\
outerjoin(Service).group_by(Waiter).all()
It returns something like this:
[(<Waiter 1>, 0)]