Mongoose - projection with $elemMatch on nested fields - mongodb

Question

I'm relatively new to MongoDB/Mongoose and I've only performed simple queries. Now I'm having some trouble trying to filter my database in a slightly more complex way. I already did some research to tackle my previous issues, but now I can't move forward. Here's what happening:
This is my schema:
const userSchema = new mongoose.Schema({
email: String,
password: String,
movies: [
{
title: String,
movieId: Number,
view_count: Number,
rating: Number,
review: String,
},
],
lists: {
watched_movies: [
{
title: String,
director: String,
genres: [{ type: String }],
runtime: Number,
date: Date,
},
],
},
});
I want to make a GET request that matches simultaneously "lists.watched_movies": { _id: req.params.entryId } and also "movies.title": req.body.title for a given email, so that the outcome of the findOne query would be just those elements and not the whole document. What I'm trying to accomplish is something like that:
{
email: "some.email#gmail.com",
movies: [
{
title: "Mongoose Strikes Back",
movieId: 123,
view_count: 1,
rating: 3,
review: "Very confusing movie!"
}
],
lists: {
watched_movies: [
{
_id: 4321
title: "Mongoose Strikes Back",
director: "Mongo",
genres: ["Drama"],
runtime: 150,
date: "2021-11-22"
}
]
}
}
My first attempt to tackle it, however, wasn't successful. Here's what I tried:
router.route("/:entryId").get((req, res) => {
User.findOne(
{ email: "some.email#gmail.com" },
{
"lists.watched_movies": { $elemMatch: { _id: req.params.entryId } },
movies: { $elemMatch: { title: req.body.title } },
},
(err, entry) => {
if (!err) {
res.send(entry);
console.log(entry);
} else {
console.log(err);
}
}
);
});
It says that Cannot use $elemMatch projection on a nested field. I thought that maybe I can solve it by changing my schema, but I'd like to avoid it if possible.

Answer 1

For your scenario, you can use $filter to filter document(s) in nested array field.
db.collection.find({
email: "some.email#gmail.com"
},
{
"lists.watched_movies": {
"$filter": {
"input": "$lists.watched_movies",
"cond": {
"$eq": [
"$$this._id",
4321// req.params.entryId
]
}
}
},
movies: {
$elemMatch: {
title: "Mongoose Strikes Back"// req.body.title
}
}
})
Sample Mongo Playground