mongoose schema check if model instance is saved for the first time

Question

I have 3 schemas, 2 of which have a .pre('save') hook to push its _id into the previous one's. You can take the example of a forum with topics, questions and comments

var topicSchema = new Schema({
  arr: {type:[Schema.ObjectId], ref:'Question'},
});
var Topic = new mongoose.model('Topic', topicSchema);

var questionSchema = new Schema({
  targetId: {type:Schema.ObjectId, ref:'Topic', required:true},
  arr: {type:[Schema.ObjectId], ref:'Comment'},
});
var Question = new mongoose.model('Question', questionSchema);

var commentSchema = new Schema({
  targetId: {type:Schema.ObjectId, ref:'Question', required:true},
});
var Comment = new mongoose.model('Comment', commentSchema);

Now for the above schema I want: When I save a question I want to automatically push the question's _id to its respective Topic.arr and when I save a comment to auto push its it to its respective Question.arr

I've tried solving this using .pre('save') hooks as follows:

function addPreSave (schema, idProperty, containerProperty) {
  var modelName = schema.paths[idProperty].options.ref;
  var model = mongoose.models[modelName];

  schema.pre('save', function (next) {
    model.findById(this[idProperty], function (err, doc) {
      doc[containerProperty].push(this._id);
      doc.save(next);
    }.bind(this));
  });
}

addPreSave(questionSchema, 'targetId', 'arr');
addPreSave(commentSchema, 'targetId', 'arr');

All that the above function does (and the 2 calls to it) is to add a .pre('save') hook to each of the 2 schemas to add their _id in their respective parent.

---

THE ISSUE: The problem is that now, every time I save a comment, it pushes its id to the topic, but I only actually want to do this the very first time only. So in the above example if you create a topic, then a question for it and then a comment for the question, the Topic.arr will have 2 ids in it (twice the question's _id) as it saves once for the question and a second time it's triggered by the comment's call to save the question

Does anyone know a way around this, or more specifically if you know how to figure out in the pre-save hook if this is a first-time save or not?

Answer 1

So it turns out that you can find the answer to anything if you are willing to go on the second page of Google!

There is a property defined for every document called isNew which does what you might imagine. So just adding a if (!this.isNew) return next() in the method fixes the issue.