pony.orm.select(u for u in User if u.available and u.friends > 0 and ...)

from datetime import datetime, timedelta
from pony.orm import *

db = Database('sqlite', ':memory:')

class User(db.Entity):
    username = Required(str, unique=True)
    password = Required(str)
    friends = Set("User", reverse='friends')  # many-to-many symmetric relation
    online = Required(bool, default=False)    # if user is currently online
    last_visit = Optional(datetime)           # last visit time
    disabled = Required(bool, default=False)  # if user is disabled by administrator

sql_debug(True)
db.generate_mapping(create_tables=True)

def active_users():
    return User.select(lambda user: not user.disabled)

def active_users():
    return select(u for u in User if not user.disabled)

users = active_users().filter(lambda user: user.name.startswith('A'))
                      .order_by(User.name)[:10]

def recent_users(days=1):
    return active_users().filter(lambda u: u.last_visit > datetime.now() - timedelta(days))

def users_with_at_least_n_friends(n=1):
    return active_users().filter(lambda u: count(u.friends) >= n)

def online_users():
    return User.select(lambda u: u.online)

def online_users_with_at_least_n_online_friends(n=1):
    return online_users().filter(lambda u: count(f for f in u.friends if f.online) >= n)

users = online_users_with_at_least_n_online_friends(n=10)
            .order_by(User.name)[:10]

class User(db.Entity):
    username = Required(str, unique=True)
    ...
    @classmethod
    def name_starts_with(cls, prefix):
        return cls.select(lambda user: not user.disabled
                                       and user.name.startswith(prefix))

...
users = User.name_starts_with('A').order_by(desc(User.last_visit))[:10]

def select_active(cls):
    return cls.select(lambda obj: not obj.deleted)

select_active(Message).filter(lambda msg: msg.author == current_user)

def active_users():
    return User.select(lambda user: not user.disabled)

def name_starts_with(query, prefix):
    return query.filter(lambda user: user.name.startswith('prefix'))

users1 = name_starts_with(active_users(), 'A').order_by(User.last_visited)
users2 = name_starts_with(recent_users(), 'B').filter(lambda user: user.online)

select(u for u in User).recent(days=3).name_starts_with('A')[:10]