Mongoose populate method not populating all fields after updating document - mongodb

Question

I change the order of an array and then populate doesn't work. It still works on the documents that I did not try to update.
I have a seccion model with preguntas inside of it
SECCION MODEL
const Schema = mongoose.Schema;
const seccionSchema = new Schema({
titulo: { type: String },
preguntas: [
{
type: Schema.Types.ObjectId,
ref: 'Pregunta',
},
],
});
PREGUNTA MODEL
const Schema = mongoose.Schema;
const preguntaSchema = new Schema({
titulo: { type: String, required: true },
});
A "Seccion" document with 2 "preguntas" looks something like this:
{
"preguntas": [
"5fb0a48ccb68c44c227a0432",
"5fb0a48ccb68c44c227a0433"
],
"_id": "5fb0a50fcb68c44c227a0436",
"titulo": "Seccion 2",
"__v": 3
}
What I want to do is change the order in the preguntas array, so that the _ids end up being [33, 32] instead of [32, 33] (last two digits of the _ids). I can update using ".save()" method or ".findOneAndUpdate()". Here is the code using "findOneAndUpdate":
router.put('/:id', async (req, res) => {
const seccionId = req.params.id;
try {
const seccion = await Seccion.findOneAndUpdate(
{ _id: seccionId },
{ preguntas: ['5fb0a48ccb68c44c227a0433', '5fb0a48ccb68c44c227a0432'] },
{ new: true }
);
res.json(seccion);
} catch (err) {
res.json({ message: err });
}
});
The result works (and the order is changed in mongo atlas):
{
"preguntas": [
"5fb0a48ccb68c44c227a0433",
"5fb0a48ccb68c44c227a0432"
],
"_id": "5fb0a50fcb68c44c227a0436",
"titulo": "Seccion 2",
"__v": 4
}
But when I try to use the "populate()" method of mongoose, only one of the "preguntas ids" or none are populated (below only the "pregunta" with id ending in 32 was populated):
{
"preguntas": [
{
"_id": "5fb0a48ccb68c44c227a0432",
"titulo": "Pregunta 3",
"__v": 0
}
],
"_id": "5fb0a50fcb68c44c227a0436",
"titulo": "Seccion 2",
"__v": 4
}
So my guess is that the populate method is not working correctly, but it does work on the other "Seccion" documents where I have not tryed to change the order of the "preguntas" inside the array. Here is the code that uses the "populate" method:
router.get('/:id', async (req, res) => {
const seccionId = req.params.id;
try {
const seccion = await Seccion.findById(seccionId).populate('preguntas');
res.json(seccion);
} catch (err) {
res.json({ message: err });
}
});
It has been really hard to make the populate work, I really appreciate anyone's help

Answer 1

Are you sure that pregunta of id 5fb0a48ccb68c44c227a0433 exists in the preguntas collection?
The way populate works is that it does a second find operation on the preguntas collection with the ids in the array, if a document is not found, it will be omitted from the array.
The issue is probably that the one ending with 33 does not exist in the preguntas collection.