HOWTO get list of model instances that have no tags assigned?

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Let's say I have a model, named MyModel, of which some instances have tags 
assigned.  Is there a way, using the django-tagging manager classes, to get a 
list of MyModel instances with no tags assigned?  

Or must I write my own custom queries against the django-tagging models?

It seems this would be possible to do in my own code if I could call 
TagManager._get_usage() and have it return a QuerySet rather than a list, 
because I could then apply my own filters to the returned QuerySet.

Original issue reported on code.google.com by [email protected] on 30 May 2012 at 11:50