Fixed #20988 -- Added model meta option select_on_save

The option can be used to force pre 1.6 style SELECT on save behaviour.
This is needed in case the database returns zero updated rows even if
there is a matching row in the DB. One such case is PostgreSQL update
trigger that returns NULL.

Reviewed by Tim Graham.

Refs #16649

django/db/models/base.py

@@ -667,7 +667,9 @@ class Model(six.with_metaclass(ModelBase)):
             base_qs = cls._base_manager.using(using)
             values = [(f, None, (getattr(self, f.attname) if raw else f.pre_save(self, False)))
                       for f in non_pks]
-            updated = self._do_update(base_qs, using, pk_val, values, update_fields)
+            forced_update = update_fields or force_update
+            updated = self._do_update(base_qs, using, pk_val, values, update_fields,
+                                      forced_update)
             if force_update and not updated:
                 raise DatabaseError("Forced update did not affect any rows.")
             if update_fields and not updated:
@@ -691,21 +693,27 @@ class Model(six.with_metaclass(ModelBase)):
                 setattr(self, meta.pk.attname, result)
         return updated
 
-    def _do_update(self, base_qs, using, pk_val, values, update_fields):
+    def _do_update(self, base_qs, using, pk_val, values, update_fields, forced_update):
         """
         This method will try to update the model. If the model was updated (in
         the sense that an update query was done and a matching row was found
         from the DB) the method will return True.
         """
+        filtered = base_qs.filter(pk=pk_val)
         if not values:
             # We can end up here when saving a model in inheritance chain where
             # update_fields doesn't target any field in current model. In that
             # case we just say the update succeeded. Another case ending up here
             # is a model with just PK - in that case check that the PK still
             # exists.
-            return update_fields is not None or base_qs.filter(pk=pk_val).exists()
-        else:
-            return base_qs.filter(pk=pk_val)._update(values) > 0
+            return update_fields is not None or filtered.exists()
+        if self._meta.select_on_save and not forced_update:
+            if filtered.exists():
+                filtered._update(values)
+                return True
+            else:
+                return False
+        return filtered._update(values) > 0
 
     def _do_insert(self, manager, using, fields, update_pk, raw):
         """

django/db/models/options.py

@@ -22,7 +22,8 @@ DEFAULT_NAMES = ('verbose_name', 'verbose_name_plural', 'db_table', 'ordering',
                  'unique_together', 'permissions', 'get_latest_by',
                  'order_with_respect_to', 'app_label', 'db_tablespace',
                  'abstract', 'managed', 'proxy', 'swappable', 'auto_created',
-                 'index_together', 'app_cache', 'default_permissions')
+                 'index_together', 'app_cache', 'default_permissions',
+                 'select_on_save')
 
 @python_2_unicode_compatible
 class Options(object):
@@ -35,6 +36,7 @@ class Options(object):
         self.ordering = []
         self.unique_together = []
         self.index_together = []
+        self.select_on_save = False
         self.default_permissions = ('add', 'change', 'delete')
         self.permissions = []
         self.object_name, self.app_label = None, app_label