code: def nullify(cls, ref_cls, field, frames):
          
          from mongoframes.queries import to_refs
          ids = [to_refs(f) for f in frames]
          ref_cls.get_collection().update_many(
              {field: {'$in': ids}},
              {'$set': {field: None}}
              )
docs: Nullify a reference field (does not emit signals)