docs: function(collection, options, cb) {
      var ids = [];
      var keys = _.isFunction(collection.url) ? collection.url() : collection.url;
      keys += options.keys;
      debug('findKeys', keys);
      this.store().getItem(keys, function(err, data) {
        if (data) ids.push(keys);
        cb(null, ids);
      });
    }