docs: function parseFilter(model, filter) {
    let merged = null;
    if (util.isEmptyObject(model.def.filter)) {
      merged = filter;
    } else if (util.isEmptyObject(filter)) {
      merged = model.def.filter;
    } else {
      merged = { $and: [model.def.filter, filter] };
    }
    return builder.normFilter(model, merged);
  }