Query with aggregate, lookup and pipeline in Mongodb - json

Question

I have to manage a wall with comments, each wall have many parent comments and each parent comment have child comments.
my collection walls is like this
groupId : {type: Schema.Types.ObjectId, ref: 'groups', unique: true},
comments : [{
commentId : {type: Schema.Types.ObjectId, ref: 'comments'},
user : {type: Schema.Types.ObjectId, ref: 'users'},
}],
and the collection comments is like this
text : String,
parentCommentId : {type: Schema.Types.ObjectId, ref: 'comments', default : null},
I want to display my wall by parent comments, each child comment under its parent comment.
i tried this query but it didn't return any results
db.getCollection('walls').aggregate([
{$match: {groupId: ObjectId("5e8c5caa75b1cd342a1175eb")}},
{
"$lookup": {
from: "comments",
let: { item: "$comments.commentId" },
pipeline: [
{ $match:
{ $expr: { $eq: [ "$parentCommentId", "$$item" ] }
}
},
{ $project: {
"_id": 1,
"parentCommentId": 1,
"text": 1
} }
],
as: "comments"
}
},
{
$project: {
groupId: 1,
"comments":1,
date: 1
}
}
])
data in walls
{
"_id" : ObjectId("5e95b4b49d3e303d667a8b71"),
"groupId" : ObjectId("5e8c5caa75b1cd342a1175eb"),
"comments" : [
{
"_id" : ObjectId("5e95b4b49d3e303d667a8b72"),
"commentId" : ObjectId("5e95b4b49d3e303d667a8b70")
},
{
"_id" : ObjectId("5e95b4ef80ae1244693aa857"),
"commentId" : ObjectId("5e95b4ef80ae1244693aa856")
},
{
"_id" : ObjectId("5e95b51080ae1244693aa859"),
"commentId" : ObjectId("5e95b51080ae1244693aa858")
},
{
"_id" : ObjectId("5e95b51d80ae1244693aa85b"),
"commentId" : ObjectId("5e95b51d80ae1244693aa85a")
},
{
"_id" : ObjectId("5e95b53580ae1244693aa85e"),
"commentId" : ObjectId("5e95b53580ae1244693aa85c")
}
],
}
data in comments
{
"_id" : ObjectId("5e95b4b49d3e303d667a8b70"),
"parentCommentId" : null,
"text" : "Hello parent 1"
}
{
"_id" : ObjectId("5e95b4ef80ae1244693aa856"),
"parentCommentId" : null,
"text" : "Hello parent 2",
"date" : ISODate("2020-04-14T13:04:47.860Z")
}
{
"_id" : ObjectId("5e95b51080ae1244693aa858"),
"parentCommentId" : ObjectId("5e95b4b49d3e303d667a8b70"),
"text" : "Hello child 1 parent 1"
}
{
"_id" : ObjectId("5e95b51d80ae1244693aa85a"),
"parentCommentId" : ObjectId("5e95b4b49d3e303d667a8b70"),
"text" : "Hello child 2 parent 1"
}
{
"_id": "5e95b53580ae1244693aa85c",
"parentCommentId": "5e95b4ef80ae1244693aa856",
"text": "Hello child 1 parent 2",
}
desired result
{
"success": true,
"data": [
{
"_id": "5e95b4b49d3e303d667a8b71",
"groupId": "5e8c5caa75b1cd342a1175eb",
"comments": [
{
"_id": "5e95b4b49d3e303d667a8b70",
"parentCommentId": null,
"text": "Hello parent 1",
"childs": {
{
"_id": "5e95b51080ae1244693aa858",
"parentCommentId": "5e95b4b49d3e303d667a8b70",
"text": "Hello child 1 parent 1",
},
{
"_id": "5e95b51d80ae1244693aa85a",
"parentCommentId": "5e95b4b49d3e303d667a8b70",
"text": "Hello child 2 parent 1",
},
}
},
{
"_id": "5e95b4ef80ae1244693aa856",
"parentCommentId": null,
"text": "Hello parent 2",
"childs": {
{
"_id": "5e95b53580ae1244693aa85c",
"parentCommentId": "5e95b4ef80ae1244693aa856",
"text": "Hello child 1 parent 2",
}
}
},
],
}
]
}
How can i modify my query ? thank you.

Answer 1

You can use below aggregation
db.walls.aggregate([
{ "$lookup": {
"from": "comments",
"let": { "commentIds": "$comments.commentId" },
"pipeline": [
{ "$match": {
"$expr": { "$in": ["$_id", "$$commentIds"] },
"parentCommentId": null
}},
{ "$sort": { "text": -1 }},
{ "$graphLookup": {
"from": "comments",
"startWith": "$_id",
"connectFromField": "parentCommentId",
"connectToField": "parentCommentId",
"as": "childs"
}}
],
"as": "comments"
}}
])
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