code: function initRelations (args) {
      // create map of relations
      this.relations = {}
      // get relations from args
      var relations = requireValidOptionalObject(args.relations)
      // validate and add relations
      _.each(relations, (relation, relatedModelName) => {
          this.relations[relatedModelName] = relation
      })
  }
docs: @function initRelations
  
  add relations
  
  @param {object} args
  
  @throws {Error}