code: function(index, updates) {
        var that = this;
  
        return this.readByIndex(index).then(function(doc) {
          return that._updateDoc(doc, updates);
        });
      }
docs: Update a document in the database by auto-index
  
  @param {Number|String} index
  @param {Object} updates - updates to apply to document
  @return {Promise}