Fixed exception message when resolving model_attr

This fixes the error message displayed when model_attr references an
unknown attribute.

Thanks to @wicol for the patch

Closes #1094

AUTHORS

@@ -93,3 +93,5 @@ Thanks to
     * @Xaroth for updating the app loader to be compatible with Django 1.7
     * Jaroslav Gorjatsev (jarig) for a bugfix with index_fieldname
     * Dirk Eschler (@deschler) for app loader Django 1.7 compatibility fixes
+    * Wictor (wicol) for a patch improving the error message given when model_attr references a non-existent
+      field

haystack/fields.py

@@ -84,23 +84,23 @@ def prepare(self, obj):
 
             for attr in attrs:
                 if not hasattr(current_object, attr):
-                    raise SearchFieldError("The model '%s' does not have a model_attr '%s'." % (repr(obj), attr))
+                    raise SearchFieldError("The model '%s' does not have a model_attr '%s'." % (repr(current_object), attr))
 
                 current_object = getattr(current_object, attr, None)
 
                 if current_object is None:
                     if self.has_default():
                         current_object = self._default
                         # Fall out of the loop, given any further attempts at
-                        # accesses will fail misreably.
+                        # accesses will fail miserably.
                         break
                     elif self.null:
                         current_object = None
                         # Fall out of the loop, given any further attempts at
-                        # accesses will fail misreably.
+                        # accesses will fail miserably.
                         break
                     else:
-                        raise SearchFieldError("The model '%s' has an empty model_attr '%s' and doesn't allow a default or null value." % (repr(obj), attr))
+                        raise SearchFieldError("The model '%s' combined with model_attr '%s' returned None, but doesn't allow a default or null value." % (repr(obj), self.model_attr))
 
             if callable(current_object):
                 return current_object()