util.loopKeys(this.__proto__._parents._belongsLinks, (belongsLinks, key) => { // eslint-disable-line
      for (let i = 0, ii = belongsLinks[key].length; i < ii; ++i) {
        let parentDoc = belongsLinks[key][i].doc;
        let field = belongsLinks[key][i].key;
        for (let j = 0, jj = parentDoc[field].length; j < jj; ++j) {