eagerKeys(response) {
      const key = this.isInverse() && !this.isThrough() ? this.key('foreignKey') : this.parentIdAttribute;
      return _.reject(
        _(response)
          .map(key)
          .uniq()
          .value(),
        _.isNil
      );
    }
Fetches all eagerly loaded foreign keys from the current relation.

@param {object} response
@return {array} The requested eager keys.