Metaclass that adds couter methods for reverse foreign keys of Django models.

models.py

class ForeignCountMeta(ModelBase):
    """
    Metaclass that adds couter methods for reverse foreign keys of Django models.

    The methods that will be creates are defined in the models Meta options:

    class CountedParent(models.Model, metaclass=ForeignCounterMeta):
        class Meta:
            counted_fields = {
                "child1_count": {'relation_name': 'child1', 'short_description': 'Child 1 count'},
                "child2_count": {'relation_name': 'child2', 'short_description': 'Child 1 count'},
            }

    class Child1(models.Model):
        parent = ForeignKey(to=CountedParent)

    class Child2(models.Model):
        parent = ForeignKey(to=CountedParent)

    With a CountedParent instance:
    
    print(counted_parent.child1_count())
    print(counted_parent.child2_count())
    """
    def __init__(cls, name, bases, nmspc):
        # check if the Meta options are available, not sure if this is necessary
        if cls._meta and cls._meta.counted_fields:

            def make_count(rel_name):
                """ create a counter method for a specific relation """
                def _count(self):
                    # results in "return self.rel_name_set.count()"
                    return getattr(self, "%s_set" % rel_name).count()
                return _count

            # do it for all items in the counted_fields option
            for method_name, args in cls._meta.counted_fields.items():
                # remove "relation_name" from the attributes, see further down
                relation_name = args.pop('relation_name')
                
                # create the counter method
                count_method = make_count(relation_name)
                
                # set the remaining attributes as attributes on the new method:
                for attr, value in args.items():
                    setattr(count_method, attr, value)
                setattr(cls, method_name, count_method)

        super(ForeignCountMeta, cls).__init__(name, bases, nmspc)

You have to add the counted_fields attribute to the Meta-Options in your settings.py:

import django.db.models.options as options
options.DEFAULT_NAMES = options.DEFAULT_NAMES + ('counted_fields', )