Zach Cochran
by Zach Cochran
1 min read



Really not feeling it today. Like at all.

Went through another section of Mongo training. This one actually talked about a pretty interesting topic of how to handle relationships within mongodb. So this means being able to related documents within different collections to one another. I honestly didn’t even know that was a thing.

Today’s going to be another note dump day. Forgive me:

Referencing between collections

  • Setting up references between documents between collections (think relational DB)
  • Assign an object with type Schema.Types.ObjectId and a ref equal to a model name:
    const BlogPostSchema = new Schema({
     title: String,
     content: String,
     comments: [{
         type: Schema.Types.ObjectId,
         ref: 'comment'
  • This ref value MUST match what’s being defined in model, as shown below:
    const Comment = mongoose.model('comment', CommentSchema);
  • Mongoose does magic behind the scenes when associating the data. In the following steps, Mongoose is actually handling the id referencing:
    joe = new User({name: 'Joe'});
    blogPost = new BlogPost({
     title: 'JS is great',
     content: 'Sure it is'
    comment = new Comment({
     content: 'This is a comment'
    comment.user = joe;
  • When doing a query, you can add on a populate modifier to fill in the data for a specific field:
    it.only('saves a relation between a user and a blog post', (done) => {
     User.findOne({ name: 'Joe' })
         .then((user) => {
  • There is no way to recursively crawl like this. You only get one.
    • Mongoose will not allow it
  • … But evidently there is a way to do it? Not sure why it was said that it wasn’t possible:
    it.only('saves a full relation tree', (done) => {
             path: 'blogPosts',
             populate: {
                 path: 'comments',
                 model: 'comment',
                 populate: {
                     path: 'user',
                     model: 'user'
         .then((user) => {
             assert( === 'Joe');
             assert(user.blogPosts[0].title === 'JS is great');
             assert(user.blogPosts[0].comments[0].content === 'This is a comment');
             assert(user.blogPosts[0].comments[0] === 'Joe');