How to group query result by a specific field in MongoDB without mutating the object? - mongodb

Question

I have a sample collection containing users and their departments
db.user.insertMany([
{
"department":"IT",
"firstName":{
"en":"Frodo",
"ru":"Фродо"
},
"lastName":{
"en":"Baggins",
"ru":"Беггинс"
}
},
{
"department":"IT",
"firstName":{
"en":"Bilbo",
"ru":"Билбо"
},
"lastName":{
"en":"Baggins",
"ru":"Беггинс"
}
},
{
"department":"Marketing",
"firstName":{
"en":"Merry",
"ru":"Мери"
},
"lastName":{
"en":"Brandybuck",
"ru":"Брендибак"
}
},
{
"department":"Marketing",
"firstName":{
"en":"Pippin",
"ru":"Пипин"
},
"lastName":{
"en":"Took",
"ru":"Тук"
}
}
])
Now what I want to do, is to query them grouped by the department, and apply localization.
So the desired output is
{
"IT":[
{
"_id":"5e808532ed036001cb7fe366",
"firstName":"Frodo",
"lastName":"Baggins"
},
{
"_id":"5e808532ed036001cb7fe367",
"firstName":"Bilbo",
"lastName":"Baggins"
}
],
"Marketing":[
{
"_id":"5e808532ed036001cb7fe368",
"firstName":"Merry",
"lastName":"Brandybuck"
},
{
"_id":"5e808532ed036001cb7fe369",
"firstName":"Pippin",
"lastName":"Took"
}
]
}
I have the following query which applies localization
db.user.aggregate([
{
$project: {
_id: '$_id',
firstName: '$firstName.en',
lastName: '$lastName.en'
}
}
]).pretty()
I looked through the $group documentation, but it seems to require an aggregate function, which is not what I need. How should I group the data?

Answer 1

You have to use $group.
db.collection.aggregate([
{
$group: {
_id: "$department",
data: {
$push: {
_id: "$_id",
firstName: "$firstName",
lastName: "$lastName"
}
}
}
},
{
$group: {
_id: null,
data: {
$push: {
k: "$_id",
v: "$data"
}
}
}
},
{
$replaceRoot: {
newRoot: {
$arrayToObject: "$data"
}
}
}
])
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