code: function(parent, key) {
          this.parent = this.parent || parent;
          this.key = this.key || key;
          if (this.parent !== parent) {
            throw new Error(
              'Internal Error. Relation retrieved from two different Objects.'
            );
          }
          if (this.key !== key) {
            throw new Error(
              'Internal Error. Relation retrieved from two different keys.'
            );
          }
        }
docs: Makes sure that this relation has the right parent and key.
  @private