code: function makeRelation (type, id, options) {
    if (!_.includes(['string', 'number'], typeof id) || !id) {
      return null
    }
  
    return {
      type: type,
      id: id
    }
  }
docs: Responsible for making a relations.
  @private
  @memberOf {Serializer}
  @param {String} type
  @param {String|Number} id
  @param {Object} options
  @return {Object|null}