Adds the given object ``obj`` to the given ``index``

def _add_to_index(index, obj):
    """Adds the given object ``obj`` to the given ``index``"""
    id_set = index.value_map.setdefault(indexed_value(index, obj), set())
    if index.unique:
        if len(id_set) > 0:
            raise UniqueConstraintError()
    id_set.add(obj.id)