code: function getItemId(_existingItem) {
      if (_.has(_existingItem, 'id')) {
        itemToUpdate.id = _existingItem.id;
        return String(_existingItem.id);
      } else {
        return itemToUpdate._localuid;
      }
    }
docs: Getting the unique ID of the item.
  
  If the existingItem has an ID assigned, then assign this to the item to update.
  
  The reason for this is that the "id" field of the record is assigned in the remote store.
  
  We must be able to relate the item saved locally (Using the _localuid field) to the remote identifier field (id)
  
  @param _existingItem
  @returns {*}