Fixed #16649 -- Refactored save_base logic

Model.save() will use UPDATE - if not updated - INSERT instead of
SELECT - if found UPDATE else INSERT. This should save a query when
updating, but will cost a little when inserting model with PK set.

Also fixed #17341 -- made sure .save() commits transactions only after
the whole model has been saved. This wasn't the case in model
inheritance situations.

The save_base implementation was refactored into multiple methods.
A typical chain for inherited save is:
save_base()
    _save_parents(self)
        for each parent:
            _save_parents(parent)
            _save_table(parent)
    _save_table(self)

1 files changed, 7 insertions, 6 deletions

diff --git a/docs/ref/models/instances.txt b/docs/ref/models/instances.txt
index 92071b8d3f..9f583c42ac 100644
--- a/docs/ref/models/instances.txt
+++ b/docs/ref/models/instances.txt
@@ -292,12 +292,13 @@ follows this algorithm:
 
 * If the object's primary key attribute is set to a value that evaluates to
   ``True`` (i.e., a value other than ``None`` or the empty string), Django
-  executes a ``SELECT`` query to determine whether a record with the given
-  primary key already exists.
-* If the record with the given primary key does already exist, Django
-  executes an ``UPDATE`` query.
-* If the object's primary key attribute is *not* set, or if it's set but a
-  record doesn't exist, Django executes an ``INSERT``.
+  executes an ``UPDATE``.
+* If the object's primary key attribute is *not* set or if the ``UPDATE``
+  didn't update anything, Django executes an ``INSERT``.
+
+.. versionchanged:: 1.6
+  Previously Django used ``SELECT`` - if not found ``INSERT`` else ``UPDATE``
+  algorithm. The old algorithm resulted in one more query in ``UPDATE`` case.
 
 The one gotcha here is that you should be careful not to specify a primary-key
 value explicitly when saving new objects, if you cannot guarantee the