MongoDB sort by join result with aggregate lookup - mongodb

Question

I have two collections user and post
> db.user.find().pretty()
{
"_id" : ObjectId("5d1473bc1b48d9309580a9de"),
"user_id" : NumberLong(1),
"region" : "US",
"is_join" : true
}
{
"_id" : ObjectId("5d1473bc1b48d9309580a9df"),
"user_id" : NumberLong(2),
"region" : "KR",
"is_join" : true
}
{
"_id" : ObjectId("5d1473bc1b48d9309580a9e0"),
"user_id" : NumberLong(3),
"region" : "US",
"is_join" : true
}
{
"_id" : ObjectId("5d1473bc1b48d9309580a9e1"),
"user_id" : NumberLong(4),
"region" : "US",
"is_join" : true
}
{
"_id" : ObjectId("5d1487fc1b48d9321ff5dc1f"),
"user_id" : NumberLong(5),
"region" : "US",
"is_join" : true
}
> db.post.find().pretty()
{
"_id" : ObjectId("5d1473bc1b48d9309580a9e2"),
"post_id" : NumberLong(1),
"user_id" : NumberLong(3),
"body" : "hi"
}
{
"_id" : ObjectId("5d1473bc1b48d9309580a9e3"),
"post_id" : NumberLong(2),
"user_id" : NumberLong(1),
"body" : "hello"
}
{
"_id" : ObjectId("5d1473bc1b48d9309580a9e4"),
"post_id" : NumberLong(3),
"user_id" : NumberLong(2),
"body" : "go"
}
{
"_id" : ObjectId("5d1473bc1b48d9309580a9e5"),
"post_id" : NumberLong(4),
"user_id" : NumberLong(4),
"body" : "python"
}
{
"_id" : ObjectId("5d14941b1b48d93314907345"),
"post_id" : NumberLong(5),
"user_id" : NumberLong(1),
"body" : "aa"
}
I want to join via mongo aggregate lookup operation.
So I made query like this.
db.user.aggregate([
{
'$match': {
'region': 'US',
}
},
{
'$lookup': {
'from': 'post',
'localField': 'user_id',
'foreignField': 'user_id',
'as': 'user'
}
},
{
'$project': {
'_id': 0,
'user.post_id': 1
}
}
])
Result
{ "user" : [ { "post_id" : NumberLong(2) }, { "post_id" : NumberLong(5) } ] }
{ "user" : [ { "post_id" : NumberLong(1) } ] }
{ "user" : [ { "post_id" : NumberLong(4) } ] }
{ "user" : [ ] }
As you know that post_id is unordered.
But I want to sort it descending.
Desired result
{ "user" : [ { "post_id" : NumberLong(1) } ] }
{ "user" : [ { "post_id" : NumberLong(2) } ] }
{ "user" : [ { "post_id" : NumberLong(4) } ] }
{ "user" : [ { "post_id" : NumberLong(5) } ] }
Even Better
{ "user" : [ 1, 2, 4, 5 }] }
How can I modify aggregate query?
Thanks.

Answer 1

You can do following to get what you want:
1) $unwind the user array
2) use $sort to sort all documents with post_id
3) use $group as the next stage in your aggregation query, and push user.post_id to form one sorted user array from all the docs:
db.user.aggregate([
{
'$match': {
'region': 'US',
}
},
{
'$lookup': {
'from': 'post',
'localField': 'user_id',
'foreignField': 'user_id',
'as': 'user'
}
},
{
$unwind : {
path : "$user",
preserveNullAndEmptyArrays : true
}
},
{
$sort : {
'user.post_id' : 1
}
},
{
$group : {
_id : null,
user : {
$push : "$user.post_id"
}
}
}
])
Read more about $sort , $unwind and $group for more information.