Mongoose mongodb, remove an element on an array
You need to remove the comment id from the array manually. The better way to do this is adding a remove middleware to your comment schema:
commentSchema.pre('remove', function (next) { User.update( { comments: this }, { $pull: { comments: this._id } }, { multi: true } ).exec(next)})But remove middlewares are only executed by calling doc.remove, and not Model.findByIdAndRemove. So you need to change a little your code (I'm using promises here for a better readability, but you can do this with callbacks):
app.delete('/index/:id/comments/:comment_id', function (req, res) { Comment.findById(req.params.comment_id) .then(function (comment) { return comment.remove() // doc.remove will trigger the remove middleware }) .then(function () { console.log('Comment successfully deleted!') return res.redirect('back') }) .catch(function (err) { console.log(err) res.redirect('/index') })})