Что-нибудь мешает добавить его в репку рядом с другими шорткатами?

from composition.base import CompositionField
from django.db import models

class ForeignCountField(CompositionField):
def init(self, model, link_back_name, link_to_foreign_name, filter={}, native=None, signal=None, verbose_name=None):
self.model = model
self.do = lambda object, foreign, signal: getattr(object, link_to_foreign_name).filter(**filter).count()
self.link_back_name = link_back_name
self.native = native or models.PositiveIntegerField(default=0, db_index=True, verbose_name=verbose_name)
self.signal = signal or (models.signals.post_save, models.signals.post_delete)

    self.internal_init(
        native = self.native,
        trigger = dict(
            on = self.signal,
            sender_model = self.model,
            do = self.do,
            field_holder_getter = self.instance_getter
        )
    )

def instance_getter(self, foreign):
    ''' Return instance getter with special check for generic relation '''
    instance = getattr(foreign, self.link_back_name)
    if not instance and self.link_back_name == 'content_object':
        instance = foreign.content_type.get_object_for_this_type(id=foreign.object_id)
    return instance