Mongoose aggregate group items by tag - node.js

Question

I'm trying to group items by tag. But I do not get what exactly I want to do.
There is schema Item (_id, name, tags) with tags is array of tags_id ( was referenced from Tag schema )
Item = new Schema({
id: Schema.ObjectId,
name: String,
tags: [ { type: Schema.ObjectId, ref: 'Tag' }]
});
Tag = new Schema({
id: Schema.ObjectId,
name: String
});
This is what I have done so far:
Item.aggregate([
{
"$lookup": {
"from": Tag.collection.name,
"let": { "tags": "$tags" },
"pipeline": [
{
"$match": {
"$expr": { "$in": ['$_id', '$$tags'] }
}
},
{
"$project": {
"_id": 1,
"name": 1
}
}
],
"as": 'tags'
}
},
{
"$group": {
"_id": "$tags._id",
"items": {
"$push": {
"id": "$_id",
"name": "$name",
}
}
}
}
]);
This is what I get
{
"data": [
{
"_id": [
"5eb95e8dcae79713f1de0a27"
],
"items": [
{
"id": "5eb95e9fcae79713f1de0a28",
"name": "My Item 1"
}
]
},
{
"_id": [
"5eb9564dc4317411fe79e1bf"
],
"items": [
{
"id": "5eb95b1430f138131ed90f4f",
"name": "My Item 2"
},
{
"id": "5eb95ed0cae79713f1de0a29",
"name": "My Item 3"
}
]
}
]
}
I would like to get the name of each tag, and not only the _id possibly not within an array. Below the result that I would like to receive:
{
"data": [
{
"_id": "5eb95e8dcae79713f1de0a27",
"name": "My Tag name 1",
"items": [
{
"id": "5eb95e9fcae79713f1de0a28",
"name": "My Item 1"
}
]
},
{
"_id": "5eb9564dc4317411fe79e1bf",
"name": "My Tag name 2",
"items": [
{
"id": "5eb95b1430f138131ed90f4f",
"name": "My Item 2"
},
{
"id": "5eb95ed0cae79713f1de0a29",
"name": "My Item 3"
}
]
}
]
}
What am I doing wrong?

Answer 1

One way to to this is to add the tag-name to your $group operator:
...
{
"$group": {
"_id": "$tags._id",
"tagName": {$first: "$tags.name"},
"items": {
"$push": {
"id": "$_id",
"name": "$name",
}
}
}
}
...
Note that this will take the first matching entry for each group. Another option would be to use $replaceRoot and the $mergeObjects operator, like it was done here.