Preliminary ducktyping of ManyToManyField using django-orm-extensions' ArrayField and storing a list of PKs.

datastructures.py

class QueryList(list):
    """
    A list of primary keys for a specified model which can act as a QuerySet
    
    Takes a ``model`` keyword argument indicating which model the PKs are
    associated with.
    """
    def __init__(self, *args, **kwargs):
        self.model = kwargs.pop('model')
        super(QueryList, self).__init__(*args, **kwargs)

    def get_empty_query_set(self):
        from django.db.models.query import EmptyQuerySet
        return EmptyQuerySet(self.model)

    # TODO: We should have access to the parent instance so that we can
    #       use a subquery.
    def get_query_set(self):
        # TODO: Importing this at the module level is causing DB settings
        #       to be loaded before local_settings are applied
        from django.db.models.query import QuerySet
        return QuerySet(self.model).filter(pk__in=self)

    def none(self):
        return self.get_empty_query_set()

    def all(self):
        return self.get_query_set()

    def count(self):
        return self.get_query_set().count()

    def distinct(self, *args, **kwargs):
        return self.get_query_set().distinct(*args, **kwargs)

    def filter(self, *args, **kwargs):
        return self.get_query_set().filter(*args, **kwargs)

    def exclude(self, *args, **kwargs):
        return self.get_query_set().exclude(*args, **kwargs)

    def iterator(self, *args, **kwargs):
        return self.get_query_set().iterator(*args, **kwargs)

    def order_by(self, *args, **kwargs):
        return self.get_query_set().order_by(*args, **kwargs)

    def exists(self, *args, **kwargs):
        return self.get_query_set().exists(*args, **kwargs)

fields.py

from django_orm.postgresql.fields.arrays import ArrayField

from .datastructures import QueryList

class ManyToManyArrayField(ArrayField):
    # NOTE: 'to' isn't really optional, but we weren't able to make South
    #       cooperate with it as a positional argument.
    def __init__(self, to=None, *args, **kwargs):
        self.to = to
        super(ManyToManyArrayField, self).__init__(self, *args, **kwargs)

    def to_python(self, value):
        if value is None:
            value = []
        return QueryList(value, model=self.to)

add_introspection_rules([
    (
        [ManyToManyArrayField],
        [],
        {
            'to': ['to', {}],
            'dbtype': ["_array_type", {"default" : "int"}]
        },
    ),
    ], ['thinkspace\.lib\.db\.fields\.ManyToManyArrayField']
)