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.

1 files changed, 18 insertions, 0 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 30e07be3a7..1e1a138714 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -475,6 +475,7 @@ class Model(six.with_metaclass(ModelBase, object)):
         that the "save" must be an SQL insert or update (or equivalent for
         non-SQL backends), respectively. Normally, they should not be set.
         """
+        using = using or router.db_for_write(self.__class__, instance=self)
         if force_insert and (force_update or update_fields):
             raise ValueError("Cannot force both insert and updating in model saving.")
 
@@ -502,6 +503,23 @@ class Model(six.with_metaclass(ModelBase, object)):
                                  "model or are m2m fields: %s"
                                  % ', '.join(non_model_fields))
 
+        # If saving to the same database, and this model is deferred, then
+        # automatically do a "update_fields" save on the loaded fields.
+        elif not force_insert and self._deferred and using == self._state.db:
+            field_names = set()
+            for field in self._meta.fields:
+                if not field.primary_key and not hasattr(field, 'through'):
+                    field_names.add(field.attname)
+            deferred_fields = [
+                f.attname for f in self._meta.fields
+                if f.attname not in self.__dict__
+                   and isinstance(self.__class__.__dict__[f.attname],
+                                  DeferredAttribute)]
+
+            loaded_fields = field_names.difference(deferred_fields)
+            if loaded_fields:
+                update_fields = frozenset(loaded_fields)
+
         self.save_base(using=using, force_insert=force_insert,
                        force_update=force_update, update_fields=update_fields)
     save.alters_data = True