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improved generic foreign key manager

Author:
carljm
Posted:
September 22, 2008
Language:
Python
Version:
1.0
Tags:
foreignkey generic manager query tuning
Score:
6 (after 6 ratings)

This is an improvement on snippet 984. Read it's description and this blog post for good explanations of the problem this solves.

Unlike snippet 984, this version is able to handle multiple generic foreign keys, generic foreign keys with nonstandard ct_field and fk_field names, and avoids unnecessary lookups to the ContentType table.

To use, just assign an instance of GFKManager as the objects attribute of a model that has generic foreign keys. Then: 

MyModelWithGFKs.objects.filter(...).fetch_generic_relations()

The generic related items will be bulk-fetched to minimize the number of queries. 

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from django.db.models.query import QuerySet
from django.db.models import Manager
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes.generic import GenericForeignKey

class GFKManager(Manager):
    """
    A manager that returns a GFKQuerySet instead of a regular QuerySet.

    """
    def get_query_set(self):
        return GFKQuerySet(self.model)

class GFKQuerySet(QuerySet):
    """
    A QuerySet with a fetch_generic_relations() method to bulk fetch
    all generic related items.  Similar to select_related(), but for
    generic foreign keys.

    Based on http://www.djangosnippets.org/snippets/984/

    """
    def fetch_generic_relations(self):
        qs = self._clone()

        gfk_fields = [g for g in self.model._meta.virtual_fields
                      if isinstance(g, GenericForeignKey)]
        
        ct_map = {}
        item_map = {}
        
        for item in qs:
            for gfk in gfk_fields:
                ct_id_field = self.model._meta.get_field(gfk.ct_field).column
                ct_map.setdefault(
                    (ct_id_field, getattr(item, ct_id_field)), {}
                    )[getattr(item, gfk.fk_field)] = (gfk.name, item.id)
            item_map[item.id] = item
            
        for (ct_id_field, ct_id), items_ in ct_map.items():
            ct = ContentType.objects.get_for_id(ct_id)
            for o in ct.model_class().objects.select_related().filter(
                id__in=items_.keys()).all():
                (gfk_name, item_id) = items_[o.id]
                setattr(item_map[item_id], gfk_name, o)
                
        return qs

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Comments

taddr (on January 22, 2009):

This definitely works. Thank you Carl!

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