How to iterate list in mongodb $lookup and pipeline - mongodb

Question

I have two collections i.e parent and chilednodes.
{
"_id" : "5e6cd8c1996ddf1c28e14505",
"parentList" : [
{
"_id" : "5e6c70e8996ddf1c28e14504",
"startDate" : "2020-02-25T14:01:58.697Z",
"active_id" : "child_vesrion_1",
"child_id" : "5e5e2cd4e972a95b6c32b5bf30"
},
{
"_id" : "5e6c70e8996ddf1c28e14506",
"startDate" : "2020-02-25T14:01:58.697Z",
"active_id" : "child_vesrion_1",
"child_id" : "5e5e2cd4e972a95b6c32b5bf31"
}
]
}
And childnodes are;
{
"_id" : "5e5e2cd4e972a95b6c32b5bf31",
"startDate" : "2020-03-25T14:01:58.697Z",
"endDate" : null,
"child_vesrion_1" : {
"childName" : "test3",
"createdDate" : "2020-02-25T14:01:58.697Z",
"text" : "test3 text",
"type" : "test3 type"
},
"child_vesrion_2" : {
"childName" : "Test4",
"createdDate" : "2020-02-25T14:01:58.697Z",
"text" : "test4 text",
"type" : "test4 type"
},
"active" : "child_vesrion_1"
},
{
"_id" : "5e5e2cd4e972a95b6c32b5bf30",
"startDate" : "2020-02-25T14:01:58.697Z",
"endDate" : null,
"child_vesrion_1" : {
"childName" : "test1",
"createdDate" : "2020-02-25T14:01:58.697Z",
"text" : "test1 text",
"type" : "test1 type"
},
"child_vesrion_2" : {
"childName" : "test2",
"createdDate" : "2020-02-25T14:01:58.697Z",
"text" : "test2 text",
"type" : "test2 type"
},
"active" : "child_vesrion_1"
}
Here is my query;
db.parent.aggregate([
{ $match: { "_id": "5e6cd8c1996ddf1c28e14505" } },
{
$lookup: {
from: "childnodes",
let: { "child_id": "$parentList.child_id", "activeid": "$parentList.active_id" },
pipeline: [
{ $match: { "$expr": { $eq: ["$_id", "$$child_id"] } } },
{
$project: {
"child_id": "$_id",
"start_date": "$startDate",
"current_version_Key": "$active",
"active_child_name": {
"$reduce": {
"input": { "$objectToArray": "$$ROOT" },
"initialValue": "",
"in": {
"$cond": [{ "$eq": ["$$this.k", "$$activeid"] },
"$$this.v.childName",
"$$value"
]
}
}
},
"text": {
"$reduce": {
"input": { "$objectToArray": "$$ROOT" },
"initialValue": "",
"in": {
"$cond": [{ "$eq": ["$$this.k", "$$activeid"] },
"$$this.v.text",
"$$value"
]
}
}
},
"type": {
"$reduce": {
"input": { "$objectToArray": "$$ROOT" },
"initialValue": "",
"in": {
"$cond": [{ "$eq": ["$$this.k", "$$activeid"] },
"$$this.v.type",
"$$value"
]
}
}
}
}
}
],
as: "finalList",
},
},
{
$project: {
parentList: 0,
},
},
]);
I am expecting results like;
{
"_id": "5e6cd8c1996ddf1c28e14505",
"finalList": [
{
"child_id": "5e5e2cd4e972a95b6c32b5bf30",
"start_date": "2020-02-25T14:01:58.697Z",
"current_version_Key": "child_vesrion_1",
"active_child_name": "test1",
"text": "test1 text",
"type": "test1 type",
},
{
"child_id": "5e5e2cd4e972a95b6c32b5bf31",
"start_date": "2020-02-25T14:01:58.697Z",
"current_version_Key": "child_vesrion_1",
"active_child_name": "test3",
"text": "test3 text",
"type": "test3 type",
}
]
}
But i am not getting anything in finalList. It is returning an empty array.
I have tried with different approaches but it didn't help me. I am bit new to mongodb, any help on this would be appreciable.

Answer 1

You were so close. Your parentList is an array, so when you define child_id and activeid inside $lookup, they are also array.
If we add $unwind before the $lookup + $group at the end, your query works as expected.
Try this one:
db.parent.aggregate([
{
$match: {
"_id": "5e6cd8c1996ddf1c28e14505"
}
},
{
$unwind: "$parentList"
},
{
$lookup: {
from: "childnodes",
let: {
"child_id": "$parentList.child_id",
"activeid": "$parentList.active_id"
},
pipeline: [
{
$match: {
"$expr": {
$eq: [
"$_id",
"$$child_id"
]
}
}
},
{
$addFields: {
child_version: {
$arrayElemAt: [
{
$filter: {
input: {
$objectToArray: "$$ROOT"
},
cond: {
$eq: [
"$$this.k",
"$$activeid"
]
}
}
},
0
]
}
}
},
{
$project: {
"_id": 0,
"child_id": "$_id",
"start_date": "$startDate",
"current_version_Key": "$active",
"active_child_name": "$child_version.v.childName",
"text": "$child_version.v.text",
"type": "$child_version.v.type"
}
}
],
as: "finalList"
}
},
{
$unwind: "$finalList"
},
{
$group: {
_id: "$_id",
parentList: {
$push: "$finalList"
}
}
}
])
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