save() method of Models does not support update_fields with non-rel backends

[ahmedtalhakhan]

Django version 1.5
Backend: Mongodb

save() call on models does not work correctly with update_fields...It still saves all the fields in a model. Looking at the code at ~/django/db/models/base.py in save_base, it seems that the param distinguishes_insert_from_update makes the code attempt inserts everytime and hence it forgets to take care of the fields specified in update_fields.. What can be the work around?