class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
class Place(db.Model):
    id = db.Column(db.Integer(), primary_key=True)
    sales = db.relationship('Sales', backref='place', lazy='dynamic')
    def regular_clients(self, time_start, time_end):
        pass
class Sales(db.Model):
    id = db.Column(db.Integer(), primary_key=True)
    place_id = db.Column(db.Integer, db.ForeignKey('place.id'))    
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    buyer = db.relationship("User", foreign_keys=[user_id])
    timestamp = db.Column(db.DateTime(timezone=False),default=datetime.utcnow)
    sum = db.Column(db.Integer, nullable=False)

def regular_clients(self, time_start, time_end):
    query = db.session.query(User).join(Sales.user_id).\
        filter(Sales.place_id == self.id,
               Sales.timestamp >= date_start, Sales.timestamp <= date_end)

amount = db.session.query(User).join(Sales.buyer).\
            filter(Sales.place_id == self.id,
                   Sales.timestamp >= time_start,
                   Sales.timestamp <= time_end).\
            group_by(User.id).\
            having(db.func.count(Sales.id) >= 5).count()