Handle null FK values

modelcluster/queryset.py

@@ -6,7 +6,7 @@
 from django.core.exceptions import FieldDoesNotExist
 from django.db.models import Model, prefetch_related_objects
 
-from modelcluster.utils import extract_field_value, get_model_field, sort_by_fields
+from modelcluster.utils import NullRelationshipValueEncountered, extract_field_value, get_model_field, sort_by_fields
 
 
 # Constructor for test functions that determine whether an object passes some boolean condition
@@ -369,6 +369,13 @@ def _build_test_function_from_filter(model, key_clauses, val):
     return constructor(model, attribute_name, val)
 
 
+def _run_test(test, obj):
+    try:
+        return test(obj)
+    except NullRelationshipValueEncountered:
+        return False
+
+
 class FakeQuerySetIterable:
     def __init__(self, queryset):
         self.queryset = queryset
@@ -438,7 +445,7 @@ def filter(self, **kwargs):
 
         clone = self.get_clone(results=[
             obj for obj in self.results
-            if all([test(obj) for test in filters])
+            if all([_run_test(test, obj) for test in filters])
         ])
         return clone
 
@@ -447,7 +454,7 @@ def exclude(self, **kwargs):
 
         clone = self.get_clone(results=[
             obj for obj in self.results
-            if not all([test(obj) for test in filters])
+            if not all([_run_test(test, obj) for test in filters])
         ])
         return clone
 

modelcluster/utils.py

@@ -1,6 +1,6 @@
 from functools import lru_cache
 from django.core.exceptions import FieldDoesNotExist
-from django.db.models import ManyToManyField, ManyToManyRel
+from django.db.models import ManyToManyField, ManyToManyRel, Model
 
 REL_DELIMETER = "__"
 
@@ -9,6 +9,10 @@ class ManyToManyTraversalError(ValueError):
     pass
 
 
+class NullRelationshipValueEncountered(Exception):
+    pass
+
+
 class TraversedRelationship:
     __slots__ = ['from_model', 'field']
 
@@ -100,17 +104,31 @@ def extract_field_value(obj, key, pk_only=False, suppress_fielddoesnotexist=Fals
     to get ``None`` values instead.
     """
     source = obj
-    for attr in key.split(REL_DELIMETER):
-        if hasattr(source, attr):
-            value = getattr(source, attr)
+    latest_obj = obj
+    segments = key.split(REL_DELIMETER)
+    for i, segment in enumerate(segments, start=1):
+        if hasattr(source, segment):
+            value = getattr(source, segment)
+            if isinstance(value, Model):
+                latest_obj = value
+            if value is None and i < len(segments):
+                raise NullRelationshipValueEncountered(
+                    "'{key}' cannot be reached for {obj} because a None value "
+                    "was encountered at {model_class}.{field_name}".format(
+                        key=key,
+                        obj=str(obj),
+                        model_class=latest_obj._meta.label,
+                        field_name=segment
+                    )
+                )
             source = value
             continue
         elif suppress_fielddoesnotexist:
             return None
         else:
             raise FieldDoesNotExist(
                 "'{name}' is not a valid field name for {model}".format(
-                    name=attr, model=type(source)
+                    name=segment, model=type(source)
                 )
             )
     if pk_only and hasattr(value, 'pk'):