Saving refs to other documents works the same way you normally save properties, just assign the _id value: code example

Example: mongodb populate document

Story.
  findOne({ title: 'Casino Royale' }).
  populate('author').
  exec(function (err, story) {
    if (err) return handleError(err);
    console.log('The author is %s', story.author.name);
    // prints "The author is Ian Fleming"
  });