docs: function constructIdCondition(_id) {
    if (typeof _id === 'number' || _id instanceof ObjectID || _id.constructor === Object) {
      // Number or ObjectID/Object instance » Return as is
      return {_id: _id};
    } else if (typeof _id === 'string') {
      // String » Convert to Object ID
      return {_id: new ObjectID(_id)};
    } else if (_id instanceof Array) {
      // Array » Return $in condition
      for (var id, $in=[], i=0; i < _id.length; i++) {
        id = _id[i];
        if (typeof id == 'number' || id instanceof ObjectID) {
          $in.push(id);
        } else if (typeof id == 'string') {
          $in.push(new ObjectID(id));
        }
      }
      return {_id: {$in: $in}};
    }
  }