[1.5.x] Fixed #14694 -- Made ``defer()`` work with reverse relations

Reverse o2o fields are now usable with defer.

Backpatch of [6ebf115206289bce8f3d86318871faac13d6e835]

1 files changed, 16 insertions, 4 deletions

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 0d79230391..9c2af10e35 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -585,17 +585,22 @@ class Query(object):
             for name in parts[:-1]:
                 old_model = cur_model
                 source = opts.get_field_by_name(name)[0]
-                cur_model = source.rel.to
+                if is_reverse_o2o(source):
+                    cur_model = source.model
+                else:
+                    cur_model = source.rel.to
                 opts = cur_model._meta
                 # Even if we're "just passing through" this model, we must add
                 # both the current model's pk and the related reference field
-                # to the things we select.
-                must_include[old_model].add(source)
+                # (if it's not a reverse relation) to the things we select.
+                if not is_reverse_o2o(source):
+                    must_include[old_model].add(source)
                 add_to_dict(must_include, cur_model, opts.pk)
             field, model, _, _ = opts.get_field_by_name(parts[-1])
             if model is None:
                 model = cur_model
-            add_to_dict(seen, model, field)
+            if not is_reverse_o2o(field):
+                add_to_dict(seen, model, field)
 
         if defer:
             # We need to load all fields for each model, except those that
@@ -2032,3 +2037,10 @@ def add_to_dict(data, key, value):
         data[key].add(value)
     else:
         data[key] = set([value])
+
+def is_reverse_o2o(field):
+    """
+    A little helper to check if the given field is reverse-o2o. The field is
+    expected to be some sort of relation field or related object.
+    """
+    return not hasattr(field, 'rel') and field.field.unique