Why are mongoose references turning into IDs? - node.js

Question

According to http://mongoosejs.com/docs/populate.html, if I set a ref property to an object, and not an ID, when getting it, I should get back an object and not an ID. I'm referring to this part of the page:
var guille = new Person({ name: 'Guillermo' });
guille.save(function (err) {
if (err) return handleError(err);
story._creator = guille;
console.log(story._creator.name);
// prints "Guillermo" in mongoose >= 3.6
// see https://github.com/LearnBoost/mongoose/wiki/3.6-release-notes
Here's my sample code:
var T1schema = new mongoose.Schema({
otherModel:{type:mongoose.Schema.Types.ObjectId, ref:"T2"}
});
var T1 = mongoose.model('T1', T1schema);
var T2schema = new mongoose.Schema({
email: {type: String},
});
var T2 = mongoose.model('T2', T2schema);
var t1 = new T1();
var t2 = new T2({email:"foo#bar.com"});
t1.otherModel = t2;
Now when I refer to t1.otherModel, it's just an ObjectId, and not a T2. For example:
console.log(t1.otherModel.email);
prints undefined instead of "foo#bar.com". Any ideas why? Note I'm using Mongoose 3.6.18 according to it's package.json.
Thanks!

Answer 1

I think your expectations here just don't match what mongoose does. The schema is a way of saying "model M's property P will always be of type T". So when you set a value, mongoose uses the schema definitions to cast the set value to the type the schema requires. Here's a little REPL session. Note setting a number property with a string value casts it to a number, but trying to store a boolean in a number field just ignores the errant value.
> var mongoose = require('mongoose')
> var schema = new mongoose.Schema({prop1: Number})
> var Model = mongoose.model('Model', schema)
> var m = new Model
> m.prop1 = 42
42
> m.prop1
42
> m.prop1 = "42"
'42'
> m.prop1
42
> m.prop1 = false
false
> m.prop1
42
So when your schema says something is going to be an ObjectId, if you give it a model instance, mongoose immediately converts it to an ObjectId in preparation for a write to the database, which is the common case. Normally if you just set that model instance, you don't need to get it back out of the parent model before saving the parent model to the database.
So the model instance getter defined properties are always going to return something compatible with the schema, and populate has to do with loading refs from the DB, but for whatever reason mongoose just doesn't work the same comparing a .populated instance with a non-populated instance. I can see why this is confusing and perhaps unexpected/disappointing though.

Answer 2

Mongoose is normalizing the instance to match the Schema, which specifies that otherModel is an ObjectId.
otherModel:{type:mongoose.Schema.Types.ObjectId, ref:"T2"}
So, Mongoose treats t1.otherModel = t2; the same as:
t1.otherModel = t2._id;
The ref isn't used until .populate() is called (either directly on the document or in a query), which needs both objects to be saved:
t2.save(function (err) {
t1.save(function (err) {
console.log(t1.otherModel);
// 7890ABCD...
t1.populate('otherModel', function () {
console.log(t1.otherModel);
// { email: 'foo#bar.com', _id: 7890ABCD..., __v: 0 }
});
});
});