constructor(config) {
    if (config.hrefType) {
      this._hrefType = config.hrefType;
      config.hrefType = undefined;
    }
    if (config.to) {
      this._to = config.to;
      config.to = undefined;
    }
    extendDefined(this, config);
    this.id = `${_.uniqueId()}`;
  }
Create a new Relation instance. For existing assets the
instantiation of relations happens automatically if you use the
`populate` transform. You only need to create relations manually
when you need to introduce new ones.

Note that the base Relation class should be considered
abstract. Please instantiate the appropriate subclass.

Options:

 - `from` The source asset of the relation.
 - `to`   The target asset of the relation, or an asset configuration
          object if the target asset hasn't yet been resolved and created.

@constructor
@param {Object} config The relation configuration