recordId (record) {
    if (record) {
      return utils.get(record, this.recordId())
    }
    return this.mapper ? this.mapper.idAttribute : this.idAttribute
  }
Return the primary key of the given, or if no record is provided, return the
name of the field that holds the primary key of records in this Collection.

@method Collection#recordId
@since 3.0.0
@param {(Object|Record)} [record] The record whose primary key is to be
returned.
@returns {(string|number)} Primary key or name of field that holds primary
key.