mezzanine.generic.fields module fix for related_model

fields.diff

diff --git a/mezzanine/generic/fields.py b/mezzanine/generic/fields.py
index 277b04d..cd3160c 100644
--- a/mezzanine/generic/fields.py
+++ b/mezzanine/generic/fields.py
@@ -36,9 +36,6 @@ class BaseGenericRelation(GenericRelation):
         attribute for the ``to`` argument.
         """
         kwargs.setdefault("object_id_field", "object_pk")
-        to = getattr(self, "related_model", None)
-        if to:
-            kwargs.setdefault("to", to)
         super(BaseGenericRelation, self).__init__(*args, **kwargs)
 
     def contribute_to_class(self, cls, name):
@@ -134,9 +131,12 @@ class CommentsField(BaseGenericRelation):
     deleted.
     """
 
-    related_model = "generic.ThreadedComment"
     fields = {"%s_count": IntegerField(editable=False, default=0)}
 
+    def __init__(self, *args, **kwargs):
+        kwargs.setdefault("to", "generic.ThreadedComment")
+        super(CommentsField, self).__init__(*args, **kwargs)
+
     def related_items_changed(self, instance, related_manager):
         """
         Stores the number of comments. A custom ``count_filter``
@@ -160,7 +160,6 @@ class KeywordsField(BaseGenericRelation):
     searching.
     """
 
-    related_model = "generic.AssignedKeyword"
     fields = {"%s_string": CharField(editable=False, blank=True,
                                      max_length=500)}
 
@@ -170,6 +169,7 @@ class KeywordsField(BaseGenericRelation):
         admin class fieldsets and pass validation, and also so that
         it shows up in the admin form.
         """
+        kwargs.setdefault("to", "generic.AssignedKeyword")
         super(KeywordsField, self).__init__(*args, **kwargs)
         self.editable = True
 
@@ -247,11 +247,14 @@ class RatingField(BaseGenericRelation):
     fields when a rating is saved or deleted.
     """
 
-    related_model = "generic.Rating"
     fields = {"%s_count": IntegerField(default=0, editable=False),
               "%s_sum": IntegerField(default=0, editable=False),
               "%s_average": FloatField(default=0, editable=False)}
 
+    def __init__(self, *args, **kwargs):
+        kwargs.setdefault("to", "generic.Rating")
+        super(RatingField, self).__init__(*args, **kwargs)
+
     def related_items_changed(self, instance, related_manager):
         """
         Calculates and saves the average rating.