fix: attribute resolution on models which have a property named all (django-haystack#1405)

Thanks to Henrique Chehad (@henriquechehad) for the patch

Closes django-haystack#1404

Author: henriquechehad

haystack/fields.py

@@ -9,6 +9,8 @@
 from haystack.exceptions import SearchFieldError
 from haystack.utils import get_model_ct_tuple
 
+from inspect import ismethod
+
 
 class NOT_PROVIDED:
     pass
@@ -110,7 +112,6 @@ def resolve_attributes_lookup(self, current_objects, attributes):
 
             if len(attributes) > 1:
                 current_objects_in_attr = self.get_iterable_objects(getattr(current_object, attributes[0]))
-
                 return self.resolve_attributes_lookup(current_objects_in_attr, attributes[1:])
 
             current_object = getattr(current_object, attributes[0])
@@ -148,7 +149,10 @@ def get_iterable_objects(cls, current_objects):
 
         if hasattr(current_objects, 'all'):
             # i.e, Django ManyToMany relationships
-            current_objects = current_objects.all()
+            if ismethod(current_objects.all):
+                return current_objects.all()
+            return []
+
         elif not hasattr(current_objects, '__iter__'):
             current_objects = [current_objects]