code: function(instance, value) {
      var collection = this._getCollectionCache(instance);
      var current = collection && collection[0];
      if (current) { this.removeObjects(instance, current); }
      if (value) { this.addObjects(instance, value); }
    }
docs: The item property setter for this relation.
  
  This property allows altering the associated object of a specific model in
  a given relation.
  
  It is accessible on an individual model via assignment with `<singular>`
  and via `set<Singular>`. For instance, a user that has one blog would
  cause this method to get triggered via
  `user.blog = '...'` or `user.setBlog`.
  
  The naming conventions are set forth in {@link HasOne#overrides}.
  
  @method
  @protected
  @param {Model} instance The model instance on which to operate.
  @see {@link BaseRelation#methods}