The model field subclass returns string generated from the list of choices.

class MultipleTypedChoiceModelField(models.Field):
    """
    Saving value of all the checked checkboxes in the string
    """    
    def get_internal_type(self):
        return 'MultipleTypedChoiceModelField'
    
    def get_prep_value(self, value):
        # In typical cases, you do not need to call additional methods,         
        # as incoming list already contains strings. 
        unicoded_val = [val.__unicode__() for val in value]
        return ', '.join(unicoded_val)