Mongoose Populate returning null or undefined

I was stuck with this problem for a week. After trying everything and making sure that I had no circular references, I decided to reset my DB.

After removing all collections, and storing them again, all my relations managed by mongoose's populate() worked.

It is so frustrating because this issue is not documented and I lost 20-30 hours trying to make a stupid collection reference work.

Since your example includes raw Object IDs not even quoted as valid javascript strings, it's hard to understand what is a mistake in your question vs a mistake in your code, but mostly your second query looks like it should work. I suspect you are mixing up your ObjectId values. You are using the same ObjectId for your story and your user and that won't work:

   author_id: mongoose.Types.ObjectId(528149b38da5b85003000002),

That's the same ID you use to lookup the story. So I think the fix is:

1. Make sure you have a valid user record in the users collection
2. Make sure your test story record's story.author_id matches that
3. Make sure you are looking up the story by the story Id and not the author_id.

If you still can't get this working, code the entire thing in a single .js file with both schemas, both models, and some code to create the records then do the queries, and post that.

Is it possible that you forget to include a "ref" field in your Schema?I just encountered this issue and after adding a ref field to my Schema it works immediately without any need to reset the database. Hope this information helps.

I found an example documented here: https://mongoosejs.com/docs/3.0.x/docs/populate.html. Although it is a documentation for version 3.0.x, I found it helpful even for version 5.5.5 which is now I am using.