Mongoose populate array of object ids

I'm trying to populate my Thread schemas GET response with all the comments related that that specific thread. I've specified a path within the Thread model to accept an array of Comments that I'll then populate when I request a thread, but it's continuing to be empty.

I'm not sure if I then need to push all the comments into the thread, but I think with the way I'm doing it, it's not required? I'm using Mongoose 4.4.19. I've followed along with the docs but still can't figure out where I've gone wrong.

Thread Schema:

const threadSchema = new Schema({
    user: {
        type: Schema.ObjectId,
        ref: 'User',
        required: true
    },
    title: {
        type: String
    },
    content: {
        type: String
    },
    category: {
        type: String
    },
    comments: [{
        type: Schema.ObjectId,
        ref: 'Comment'
    }]
}, {
    timestamps: true
})

Comment Schema:

const commentSchema = new Schema({
  user: {
    type: Schema.ObjectId,
    ref: 'User',
    required: true
  },
  thread: {
    type: Schema.ObjectId,
    ref: 'Thread',
    required: true
  },
  content: {
    type: String
  }
}, {
  timestamps: true
})

Handles get requests:

export const index = ({ querymen: { query, select, cursor } }, res, next) =>
  Thread.find(query, select, cursor)
    .populate('user comments')
    .then(threads => threads.map(thread => thread.view()))
    .then(success(res))
    .catch(next)

via Jordan