We can't use the default manager in Model.save_base(), since we need to
retrieve existing objects which might be filtered out by that manager. We now
always use a plain Manager instance at that point (or something that can
replace it, such as a GeoManager), making all existing rows in the
database visible to the saving code.
The logic for detecting a "suitable replacement" plain base is the same as for
related fields: if the use_for_related_fields is set on the manager subclass,
we can use it. The general requirement here is that we want a base class that
returns the appropriate QuerySet subclass, but does not restrict the rows
returned.
Fixed#8990, #9527.
Refs #2698 (which is not fixed by this change, but it's the first part of a
larger change to fix that bug.)
Backport of r10056 from trunk.
git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10059 bcc190cf-cafb-0310-a4f2-bffc1f526a37
97321c7645