Filters given entity queryset by an attribute which is linked to given
        many-to-many schema.

def _filter_by_m2m_schema(self, qs, lookup, sublookup, value, schema, model=None):
        """
        Filters given entity queryset by an attribute which is linked to given
        many-to-many schema.
        """
        model = model or self.model
        schemata = dict((s.name, s) for s in model.get_schemata_for_model())   # TODO cache this dict, see above too
        try:
            schema = schemata[lookup]
        except KeyError:
            # TODO: smarter error message, i.e. how could this happen and what to do
            raise ValueError(u'Could not find schema for lookup "%s"' % lookup)
        sublookup = '__%s'%sublookup if sublookup else ''
        return {
            'attrs__schema': schema,
            'attrs__choice%s'%sublookup: value,  # TODO: can we filter by id, not name?
        }