Fixed #18306 -- Made deferred models issue update_fields on save

Deferred models now automatically update only the fields which are
loaded from the db (with .only() or .defer()). In addition, any field
set manually after the load is updated on save.

2 files changed, 22 insertions, 0 deletions

diff --git a/docs/ref/models/instances.txt b/docs/ref/models/instances.txt
index 14541ad0d1..b7ae3890cf 100644
--- a/docs/ref/models/instances.txt
+++ b/docs/ref/models/instances.txt
@@ -386,6 +386,12 @@ perform an update on all fields.
 
 Specifying ``update_fields`` will force an update.
 
+When saving a model fetched through deferred model loading
+(:meth:`~Model.only()` or :meth:`~Model.defer()`) only the fields loaded from
+the DB will get updated. In effect there is an automatic ``update_fields`` in
+this case. If you assign or change any deferred field value, these fields will
+be added to the updated fields.
+
 Deleting objects
 ================
 
diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 8c188c67c3..b59b2ece82 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -1110,6 +1110,14 @@ one, doing so will result in an error.
     reader, is slightly faster and consumes a little less memory in the Python
     process.
 
+.. versionchanged:: 1.5
+
+.. note::
+
+    When calling :meth:`~Model.save()` for instances with deferred fields,
+    only the loaded fields will be saved. See :meth:`~Model.save()` for more
+    details.
+
 
 only
 ~~~~
@@ -1154,6 +1162,14 @@ All of the cautions in the note for the :meth:`defer` documentation apply to
 options. Also note that using :meth:`only` and omitting a field requested
 using :meth:`select_related` is an error as well.
 
+.. versionchanged:: 1.5
+
+.. note::
+
+    When calling :meth:`~Model.save()` for instances with deferred fields,
+    only the loaded fields will be saved. See :meth:`~Model.save()` for more
+    details.
+
 using
 ~~~~~