Add reference (docs) on how to delete a resource child object attribute on update #20

itemRef.once('value', dataSnapshot => {
  try {
    let item = dataSnapshot.val() || {};
    let fields, removed, i;
    if (resourceConfig.relations) {
      fields = resourceConfig.relationFields;
      removed = [];
      for (i = 0; i < fields.length; i++) {
        removed.push(attrs[fields[i]]);
        delete attrs[fields[i]];
      }
    }
    DSUtils.deepMixIn(item, attrs);
    if (resourceConfig.relations) {
      fields = resourceConfig.relationFields;
      for (i = 0; i < fields.length; i++) {
        let toAddBack = removed.shift();
        if (toAddBack) {
          attrs[fields[i]] = toAddBack;
        }
      }
    }
    itemRef.set(item, err => {
      if (err) {
        reject(err);
      } else {
        resolve(item);
      }
    });
  } catch (err) {
    reject(err);
}