code: function(instance, relatedObject, options) {
      var opts = _.defaults({}, options, { follow: true, attrs: true });
      var inverse = opts.follow && relatedObject &&
        relatedObject[this.inverse + 'Relation'];
  
      if (opts.attrs) {
        // set the foreign key property as well
        instance.setAttribute(this.foreignKeyAttr,
          relatedObject[this.primaryKey]);
      }
  
      this._setRelated(instance, relatedObject);
  
      if (inverse) {
        inverse.associate(relatedObject, instance, _.extend({}, options, {
          follow: false,
        }));
      }
    }
docs: Override of {@link BaseRelation#associate}.
  
  @method
  @protected
  @see {@link BaseRelation#associate}