Meteor reactive publication across collections (essentially a join)

publications.js

// on the server
Meteor.publish('comments', function(id) {
  check(id, String);
  var sub = this, userHandles = [], commentsHandle = null, userCursor;

  // send over the author for the comment
  var publishCommentAuthor = function publishCommentAuthor(authorId) {
    userCursor = Meteor.users.find({ _id: authorId});
    userHandles[authorId] = 
      Mongo.Collection._publishCursor(userCursor, sub, 'users');
  };

  commentsHandle = Comments.find({
    mediaId: id
  }, {
    sort: { createdAt: -1}
  }).observeChanges({
    added: function(id, post) {
      // publish the new user record
      publishCommentAuthor(post.createdBy);

      // add the new post to the publication
      sub.added('comments', id, post);
    },
    changed: function(id, fields) {
      // update the comment if it changed
      sub.changed('comments', id, fields);
    },
    removed: function(id) {
      // get the author
      var authorId = Comments.findOne(id).createdBy;

      // stop observing changes on the comments author
      userHandles[authorId] && userHandles[authorId].stop();

      // delete the comment
      sub.removed('comments', id);
    }
  });

  sub.ready();

  /* make sure we clean everything up (note `_publishCursor`
  does this for us with the user observers) */
  sub.onStop(function() { commentsHandle.stop(); });
});

/*
    This example gets the user objects related to comments for a post.
    the id received by the publication from the subscription is the id of the post
    that the comments belong to.  We then run a function each time a document is added
    to the comments array to get the associated user record, and leveraging the 
    '_publishCursor' method this gets added to the users collection.
*/