code: function(instance, objects) {
      var foreignKeyAttr = this.foreignKeyAttr;
      var fk = instance.getAttribute(foreignKeyAttr);
      if (fk && objects.length === 0) {
        var relatedModel = this._relatedModel;
        var relatedClass = relatedModel.__identity__;
        var relatedName = relatedClass.__name__;
        throw new Error(util.format('Found no %s with %j %d',
          relatedName, foreignKeyAttr, fk));
      }
    }
docs: Validate fetched objects, ensuring that exactly one object was obtained
  when the foreign key is set.
  
  @method
  @protected
  @param {Model} instance The model instance on which to operate.