Changeset [8536] changed the way field iexact lookups are translated in the PostgreSQL backend. Before the patch, these were translated to "ILIKE %s". Now it's "UPPER(field) = UPPER(%s)". However, the %s value is still escaped for use in a LIKE expression, that is, all underscores are prepended with "\\".
Before the patch (correct):
>>> User.objects.filter(username__iexact='test_user').query.as_sql()
('SELECT [...] FROM "auth_user" WHERE "auth_user"."username"::text ILIKE %s ', (u'test\\_user',))
After the patch (wrong):
User.objects.filter(username__iexact='test_user').query.as_sql()
('SELECT [...] FROM "auth_user" WHERE UPPER("auth_user"."username"::text) = UPPER(%s) ', (u'test\\_user',))
As a result, iexact lookups no longer work in the current version with PostgreSQL, if the comparison value contains an underscore.
