Prepare data for persistency by exporting it to native form
         and making sure we're not persisting with a null primary key.

def prepare_data(self, data=None):
        '''
        Prepare data for persistency by exporting it to native form
         and making sure we're not persisting with a null primary key.
        '''
        data = data or self.export_data(native=True)
        # make sure we don't persist a null _id and let MongoDB auto-generate it
        if '_id' in data and data['_id'] is None:
            del data['_id']
        return data