docs: function(key) {
        var that = this;
  
        if (!this._key || !key) {
          return Promise.reject(new Error('No unique key specified'));
        }
  
        var props = {};
        props[this._key] = key;
  
        return this._dbApi.find(props).then(function(docs) {
          return that._readDoc(docs[0]);
        });
      }