Split polymorphic ctype objects into groups by type.

django_polymorphic_group_by_type.py

def group_by_type(models):
    """
    Yield a sequence of query sets, where each one only has one Polymorphic ctype, this
    makes it possible to use prefetch and select_related with the resulting querysets.
    """
    pks = models.values_list("pk", flat=True)
    polymorphic_ctypes = models.non_polymorphic().distinct("polymorphic_ctype_id").values_list("polymorphic_ctype", flat=True)

    for polymorphic_ctype in polymorphic_ctypes:
        # Query contenttypes to get the concrete class instance
        klass = ContentType.objects.get_for_id(polymorphic_ctype).model_class()

        yield klass.objects.filter(pk__in=pks)

def select_related_copyable_fields(self):
    """
    Split models into separate querysets, using group_by_type and call select_related on any related fields
    found in the `copyable_fields` attribute.
    """
    pks = models.values_list("pk", flat=True)
    for qs in group_by_type(models):
        # Work out which fields to use in select_related
        related_fields = [field.name for field in qs.model.copyable_fields if hasattr(field, "related_query_name")]
        yield qs.select_related(*related_fields).filter(pk__in=pks)