diff --git a/README.md b/README.md
index 0756ada..3834210 100644
--- a/README.md
+++ b/README.md
@@ -316,7 +316,7 @@ enriched_activities = enricher.enrich_activities(activities)
 
 #### Change how models are retrieved
 
-The enrich class that comes with the packages tries to minimise the amount of database queries. The models are grouped by their class and then retrieved with a pk__in query. You can implement a different approach to retrieve the instances of a model subclassing the ```stream_django.enrich.Enrich``` class.
+The enrich class that comes with the packages tries to minimise the amount of database queries. The models are grouped by their class and then retrieved with a bulk query. You can implement a different approach to retrieve the instances of a model subclassing the ```stream_django.enrich.Enrich``` class.
 
 To change the retrieval for every model you should override the ```fetch_model_instances``` method; in alternative you can change how certain models' are retrieved by implementing the hook function ```fetch_<model_name>_instances```
 
@@ -326,10 +326,10 @@ class MyEnrich(Enrich):
     Overwrites how model instances are fetched from the database
     '''
 
-    def fetch_model_instances(self, modelClass, pks):
+    def fetch_model_instances(self, modelClass, lookup_values):
         '''
-        returns a dict {id:modelInstance} with instances of model modelClass
-        and pk in pks
+        returns a dict {lookup_value:modelInstance} with instances of model modelClass
+        and lookup_value in lookup_values
         '''
         ...
 
@@ -338,7 +338,7 @@ class AnotherEnrich(Enrich):
     Overwrites how Likes instances are fetched from the database
     '''
 
-    def fetch_like_instances(self, pks):
+    def fetch_like_instances(self, lookup_values):
         return {l.id: l for l in Like.objects.cached_likes(ids)}
 
 ```
diff --git a/stream_django/activity.py b/stream_django/activity.py
index 9499ed8..3b5da15 100644
--- a/stream_django/activity.py
+++ b/stream_django/activity.py
@@ -20,17 +20,30 @@ def create_model_reference(model_instance):
     creates a reference to a model instance that can be stored in activities
 
     >>> from core.models import Like
-    >>> like = Like.object.get(id=1)
+    >>> like = Like.object.get(<lookup_field>=1)  where lookup_field defaults to 'pk'
     >>> create_reference(like)
     core.Like:1
 
     '''
+    try:
+        field_name = model_instance.activity_lookup_field()
+        content_id = getattr(model_instance, field_name)
+    except AttributeError:
+        content_id = model_instance.pk
+
     content_type = model_content_type(model_instance.__class__)
-    content_id = model_instance.pk
     return '%s:%s' % (content_type, content_id)
 
 
 class Activity(object):
+
+    @classmethod
+    def activity_lookup_field(cls):
+
+        '''
+        The name of the field to use for model lookups 
+        '''
+        return 'pk'
     
     @property
     def activity_author_feed(self):
@@ -78,7 +91,12 @@ def activity_object_attr(self):
 
     @property
     def activity_actor_id(self):
-        return self.activity_actor_attr.pk
+        try:
+            field_name = self.activity_actor_attr.activity_lookup_field()
+            actor_id = getattr(self.activity_actor_attr, field_name)
+        except AttributeError:
+            actor_id = self.activity_actor_attr.pk            
+        return actor_id
 
     @property
     def activity_actor(self):
diff --git a/stream_django/enrich.py b/stream_django/enrich.py
index 1af2045..1c8c8e0 100644
--- a/stream_django/enrich.py
+++ b/stream_django/enrich.py
@@ -3,8 +3,10 @@
 import operator
 import itertools
 
+
 try:
     from django.apps import apps
+    from django.core.exceptions import FieldDoesNotExist
     get_model = apps.get_model
 except ImportError:
     from django.db.models.loading import get_model
@@ -91,18 +93,23 @@ def _collect_references(self, activities, fields):
             model_references[f_ct].append(f_id)
         return model_references
 
-    def fetch_model_instances(self, modelClass, pks):
+    def fetch_model_instances(self, modelClass, lookup_values):
         '''
-        returns a dict {id:modelInstance} with instances of model modelClass
-        and pk in pks
+        returns a dict {<lookup_value>:modelInstance} with instances of model modelClass
+        and lookup_value in lookup_values  (i.e., {1:<obj_1>} or {<uuid>:<obj_2>})
         '''
+        try:
+            lookup_field_name = modelClass.activity_lookup_field()
+        except AttributeError:
+            lookup_field_name = 'pk'
+        
         hook_function_name = 'fetch_%s_instances' % (modelClass._meta.object_name.lower(), )
         if hasattr(self, hook_function_name):
-            return getattr(self, hook_function_name)(pks)
+            return getattr(self, hook_function_name)(lookup_values)
         qs = modelClass.objects
         if hasattr(modelClass, 'activity_related_models') and modelClass.activity_related_models() is not None:
             qs = qs.select_related(*modelClass.activity_related_models())
-        return qs.in_bulk(pks)
+        return qs.in_bulk(lookup_values, field_name=lookup_field_name)
 
     def _fetch_objects(self, references):
         objects = defaultdict(list)
@@ -119,8 +126,17 @@ def _inject_objects(self, activities, objects, fields):
                 continue
             f_ct, f_id = activity[field].split(':')
             model = get_model(*f_ct.split('.'))
-            f_id = model._meta.pk.to_python(f_id)
-
+            
+            try:
+                lookup_field_name = model.activity_lookup_field()
+                if lookup_field_name == 'pk':
+                    f_id = model._meta.pk.to_python(f_id)
+                else:
+                    lookup_field = model._meta.get_field(lookup_field_name)
+                    f_id = lookup_field.to_python(f_id)
+            except (AttributeError, FieldDoesNotExist):
+                f_id = model._meta.pk.to_python(f_id)
+                
             instance = objects[f_ct].get(f_id)
             if instance is None:
                 activity.track_not_enriched_field(field, activity[field])