Mongodb lookup with array - mongodb

Question

I have two collections first one is
user_profile collection
const userProfileSchema = mongoose.Schema({
phone_number: {
type: String,
required: false,
},
primary_skills: [
{
skill_id: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Skill'
},
years: Number,
}
]
});
sample data
{
"phone_number":"222",
"primary_skills":[{skill_id:1,years:12},{skill_id:2,years:13}]
}
in the primary_skills the key skill_id is mapped with another collection named skills
skills collection
const skillSchema = mongoose.Schema({
name: {
type: String,
required: true,
unique:true,
},
});
sample data
[
{
id:1,
name:'php'
},
{
id:2,
name:'java'
}
]
I want to fetch all values in the user_profile collection along with the respective skills name
expected output:
{
"phone_number":"222",
"primary_skills":[{
name:"php",skill_id:1,years:12
},{
name:"java",skill_id:2,years:13}
]
}
I found a similar thread to my question MongoDB lookup when foreign field is an array of objects but it's doing the opposite of what I want
This is the query I tried
profile.aggregate([{
$lookup:{
from:'skills',
localField:'primary_skills.skill_id',
foreignField:'_id',
'as':'primary_skills'
}
}])
This works fine but it didn't contain the years key

Answer 1

You need to do it with $unwind and $group,
$unwind primary_skills because its an array and we need to lookup sub document wise
db.user_profile.aggregate([
{
$unwind: "$primary_skills"
},
$lookup to join primary_skills, that you have already did
{
$lookup: {
from: "skills",
localField: "primary_skills.skill_id",
foreignField: "id",
as: "primary_skills.name"
}
},
$unwind primary_skills.name that we have stored join result, its array and we are unwinding to do object
{
$unwind: {
path: "$primary_skills.name"
}
},
$addFields replace field name that we have object and we need only name
{
$addFields: {
"primary_skills.name": "$primary_skills.name.name"
}
},
$group by _id because we have unwind and we need to combine all documents
{
$group: {
_id: "$_id",
phone_number: {
$first: "$phone_number"
},
primary_skills: {
$push: "$primary_skills"
}
}
}
])
Playground: https://mongoplayground.net/p/bDmrOwmASn5