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.

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
 ================
 

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
 ~~~~~
 

docs/releases/1.5.txt

@@ -42,6 +42,10 @@ keyword argument ``update_fields``. By using this argument it is possible to
 save only a select list of model's fields. This can be useful for performance
 reasons or when trying to avoid overwriting concurrent changes.
 
+Deferred instances (those loaded by .only() or .defer()) will automatically
+save just the loaded fields. If any field is set manually after load, that
+field will also get updated on save.
+
 See the :meth:`Model.save() <django.db.models.Model.save()>` documentation for
 more details.