Mongo rename embedded docs field

mongoRenameEmbeddedDocsFields.js

/*
 * Given the following schema for collection 'foo':
 *  myDoc: {
 *   embeddedDocs: [
 *     {
 *       bar: 'a',
 *       baz: 'b',
 *     },
 *    {
 *       bar: 'b',
 *       baz: 'b',
 *     },
 *   ]
 * }
 *
 * Renames the following fields:
 *   'bar' --> 'kar'
 *   'baz' --> 'kaz'
 *
 * @param {object} db Your database
 * @param {string} collectionName Your collection name e.g. 'foo'
 * @param {string} embeddedDocumentsFieldName e.g. 'embeddedDocs'
 * @param {array} fields Fields that you want to rename
 *   [{ oldField: 'bar', newField: 'kar' }, { oldField: 'baz', newField: 'kaz' }];
 * @return {undefined}
 */

module.exports = (db, collectionName, embeddedDocumentsFieldName, fields) => {
  const findKey = `${embeddedDocumentsFieldName}.0`;

  db.collection(collectionName)
    .find({
      [findKey]: {$exists: true}, // check > 0 subdocuments
    })
    .snapshot() // https://stackoverflow.com/a/26050901/3003438
    .forEach(doc => {
      doc[embeddedDocumentsFieldName].forEach(embeddedDocument => {
        fields.map(({oldField, newField}) => {
          if (oldField in embeddedDocument) {
            embeddedDocument[newField] = embeddedDocument[oldField];
            try {
              Reflect.deleteProperty(embeddedDocument, oldField);
            } catch (err) {
              if (err instanceof TypeError) {
                delete embeddedDocument[
                  oldField
                ]; /* eslint prefer-reflect : 0 */
              } else {
                throw err;
              }
            }
            db.collection(collectionName).update({_id: doc._id}, doc);
          }
        });
      });
    });
};