Added method to check for self-referential sortable foreign keys.

adminsortable/utils.py

@@ -1,4 +1,4 @@
-from .models import Sortable
+from .models import Sortable, SortableForeignKey
 
 
 def get_is_sortable(objects):
@@ -9,9 +9,22 @@ def get_is_sortable(objects):
     return False
 
 
-def check_model_is_sortable(cls):
-    if cls:
-        if issubclass(type(cls), Sortable):
-            if cls.objects.count() > 1:
+def is_self_referential(cls):
+    cls_type = type(cls)
+    sortable_subclass = issubclass(cls_type, Sortable)
+    sortable_foreign_key_subclass = issubclass(cls_type, SortableForeignKey)
+    if sortable_foreign_key_subclass and not sortable_subclass:
+        return True
+    return False
+
+
+def check_model_is_sortable(cls):
+    if cls:
+        if is_self_referential(cls):
+            objects = cls.model.objects
+        else:
+            objects = cls.objects
+
+        if objects.count() > 1:
             return True
     return False