How to $lookup entries from array in MongoDB? - mongodb

Question

I have three collections, Posts, Languages and Translations. And the structure looks like this:
Post: {
id: ObjectID
title: String,
translated_versions: ObjectID
}
TranslatedVersion: {
id: ObjectID
translations: Translation[]
}
// Just an object structure in TranslatedVersion.translations, not collection
Translation: {
post: ObjectID
language: ObjectID
}
Language {
id: ObjectID
name: String
}
What I want to do is to get entry from translated versions collections, put it into my post and then for every entry in TranslatedVersion.translation I want to get post and language objects, so it will look like this:
Post {
title: "Test title",
translated_versions: {
id: 'some-id',
tranlations: [
Translation {
language: {
id: 'some-id',
name: 'Turkish'
},
post: {
id: 'some-id'
title: 'Turkish post'
}
},
Translation {
language: {
id: 'some-id',
name: 'English'
},
post: {
id: 'some-id',
title: 'English post'
}
}
]
}
}
I am using Golang and I tried to do it this way (sorry lots of lines of code):
lookup_translations_group := bson.D{
{
Key: "$lookup", Value: bson.D{
{
Key: "from", Value: Models.Translation,
},
{
Key: "localField", Value: "translations_group_id",
},
{
Key: "foreignField", Value: "_id",
},
{
Key: "as", Value: "translations",
},
},
},
}
unwind_translations_group := bson.D{
{
Key: "$unwind", Value: bson.D{
{
Key: "path", Value: "$translations",
},
{
Key: "preserveNullAndEmptyArrays", Value: true,
},
},
},
}
lookup_translations_posts := bson.D{
{
Key: "$lookup", Value: bson.D{
{
Key: "from", Value: Models.Post,
},
{
Key: "localField", Value: "translations.translations.post",
},
{
Key: "foreignField", Value: "_id",
},
{
Key: "as", Value: "translations.translations.post",
},
},
},
}
Then I run mongo.Pipeline and what I get is an object with "post" field which is an array with my translated posts:
So obviously I am doing something wrong here :( I assume I need to add another pipeline in lookup_translations_group stage, but I have no idea how to it correctly.
upd2:
Even more complex examples:
db.posts.insertMany([
{ "_id" : 1, "title": "English post", language: 1, category: 1, "translations": 1, },
{ "_id" : 2, "title": "Turkish post", langauge: 2, category: 1, "translations": 1, },
{ "_id" : 2, "title": "Finnish post", language: 3, category: 1, "translations": 1, },
])
db.languages.insertMany([
{ "_id" : 1, "name": "English", },
{ "_id" : 2, "name": "Turkish", },
{ "_id" : 3, "name": "Finnish", },
])
db.translations.insert({
"_id" : 1,
translations: [
{ language: 1, post: 1 },
{ language: 2, post: 2 },
{ language: 3, post: 3 },
]
})
db.categories.insert({
"_id" : 1,
"name": "Random category"
})
Result that I want:
{
"_id": 1,
"title": "English post",
"language": {
"_id": 1,
"name": "English"
},
"category": {
"_id": 1,
"name": "Random category"
},
"translations": {
"_id": 1,
translations: [
{
"language": {
"_id": 1,
"name": "English"
},
"post": {
"_id": 1,
"title": "English post",
"category": {
"_id": 1,
"name": "Random category"
}
}
},
{
"language": {
"_id": 2,
"name": "Turkish"
},
"post": {
"_id": 2,
"title": "Turkish post",
"category": {
"_id": 1,
"name": "Random category"
}
}
},
{
"language": {
"_id": 3,
"name": "Finnish"
},
"post": {
"_id": 3,
"title": "Finnish post",
"category": {
"_id": 1,
"name": "Random category"
}
}
},
]
}
}

Answer 1

If I understand correctly, you want something like:
db.translations.aggregate([
{$match: {_id: 1}},
{$lookup: {
from: "posts",
localField: "translations.post",
foreignField: "_id",
as: "posts"
}},
{$lookup: {
from: "languages",
localField: "posts.language",
foreignField: "_id",
as: "translations"
}},
{$project: {
data: {$map: {
input: "$posts",
in: {
post: {title: "$$this.title", _id: "$$this._id"},
language: {$arrayElemAt: [
"$translations",
{$indexOfArray: ["$translations._id", "$$this._id"]}
]}
}
}}
}},
{$replaceRoot: {
newRoot: {$mergeObjects: [
{$first: {$filter: {input: "$data", cond: {$eq: ["$$this.post._id", "$_id"]}}}},
{translations: {
_id: "$_id",
translations: {
$filter: {input: "$data", cond: {$ne: ["$$this.post._id", "$_id"]}}
}
}}
]}
}},
{$project: {_id: "$post._id", title: "$post.title", language: 1, translations: 1}}
])
See how it works on the playground example