code: function(attrs, attrsToRemove) {
      _.forEach(attrs, function(val, key) {
        // shouldRemove is either an object or a boolean
        var shouldRemove = attrsToRemove[key];
        if (_.isUndefined(shouldRemove)) {
          return;
        }
  
        // Support nested object
        if (_.isPlainObject(val) && !_.isArray(val) && _.isPlainObject(shouldRemove)) {
          return this._removeExpandableAttributes(val, shouldRemove);
        }
  
        // Make sure attribute is an object
        // Strip all nested properties except for `_id`
        if (_.isPlainObject(attrs[key]) && shouldRemove) {
          attrs[key] = _.pick(attrs[key], ['_id']);
        }
      }.bind(this));
    }
docs: Remove expandable attributes
  
  Does not work for objects embedded inside arrays
  
  @param {Object} attrs
  @param {Object} attrsToRemove