code: function(key, updates) {
        var that = this;
  
        if (!this._key || !key) {
          return Promise.reject(new Error('No unique key specified'));
        }
  
        return this.readByKey(key).then(function(doc) {
          return that._updateDoc(doc, updates);
        });
      }
docs: Update a document in the database by key
  
  @param {String} key
  @param {Object} updates - updates to apply to document
  @return {Promise}