How to aggregate array of ObjectId pairs with their relevant collection - mongodb

Question

I have a course collection in which I am allotting teachers for each subject of that course. The allotment is saved as an array of JSON please take a look at the reference doc below.
{
"_id" : ObjectId("5cc7d72d8e165005cbef939e"),
"isAssigned" : true,
"name" : "11",
"section" : "A",
"allotment" : [
{
"subject" : ObjectId("5cc3f7cc88e95a0c8e8ccd7d"),
"teacher" : ObjectId("5cbee0e37a3c852868ec9797")
},
{
"subject" : ObjectId("5cc3f80e88e95a0c8e8ccd7e"),
"teacher" : ObjectId("5cbee10c7a3c852868ec9798")
}
]
}
I am trying to match the subject and teacher fields along with their doc from two different collections. I could get them in two different array's but couldn't get them as structured in my expected output
Doc in teachers collection
{
_id: ObjectId("5cbee0e37a3c852868ec9797"),
name: "Alister"
}
Doc in subject
{
_id: ObjectId("5cc3f7cc88e95a0c8e8ccd7d"),
name: "English",
code: "EN"
}
Query I tried
Course.aggregate([
{"$match": matchQuery},
{"$lookup": {
"from": "subjects",
"localField": "allotment.subject",
"foreignField": "_id",
"as": "subjectInfo"
}
},
{"$lookup": {
"from": "teachers",
"localField": "allotment.teacher",
"foreignField": "_id",
"as": "teacherInfo"}
},
])
Output of that Query
{
isAssigned: true
name: "11"
section: "A"
subjectInfo:[
{_id: "5cc3f7cc88e95a0c8e8ccd7d", name:"English", code:"EN"}
{_id: "5cc3f80e88e95a0c8e8ccd7e", name: "Science", code:"SC"}
]
teacherInfo:[
{_id: ObjectId("5cbee0e37a3c852868ec9797"),name: "Alister"},
{ _id: ObjectId("5cbee10c7a3c852868ec9798"),name: "Frank"}
]
}
Expexted output
{
"_id" : ObjectId("5cc7d72d8e165005cbef939e"),
"isAssigned" : true,
"name" : "11",
"section" : "A",
"allotment" : [
{
"subject" : {
_id: ObjectId("5cc3f7cc88e95a0c8e8ccd7d"),
name: "English",
code: "EN"
}
"teacher" : {
_id: ObjectId("5cbee0e37a3c852868ec9797"),
name: "Alister"
}
},
{
"subject" : {
_id: ObjectId("5cc3f80e88e95a0c8e8ccd7e"),
name: "Science",
code: "SC"
}
"teacher" : {
_id: ObjectId("5cbee10c7a3c852868ec9798"),
name: "Frank"
}
}
]
}

Answer 1

Just unwind the array before the lookups:
Course.aggregate([
{"$match": matchQuery},
{"$unwind: "$allotment"}
{"$lookup": {
"from": "subjects",
"localField": "allotment.subject",
"foreignField": "_id",
"as": "subjectInfo"
}
},
{"$lookup": {
"from": "teachers",
"localField": "allotment.teacher",
"foreignField": "_id",
"as": "teacherInfo"}
},
])
if you want to re-group after that to restore expected format you can add:
{ $group : {
_id: "$_id",
name: {$first: "$name"},
section: {$first: "$section},
isAssigned: {$first: "$isAssigned},
allotment: {$push: {teacher: "$teacherInfo.0", subject: "$subjectInfo.0"}}
I'm assuming teacherInfo and subjectInfo are never empty, if this is not the case you should add a $match to filter empty ones.

Answer 2

Take a look at $lookup aggregation stage which lets you join collections. There's a plenty of examples on the usage in the documentation.
EDIT: Here's the complete pipeline that should provide the expected result:
courses.aggregate(
[
{
"$unwind" : {
"path" : "$allotment"
}
},
{
"$lookup" : {
"from" : "subjects",
"localField" : "allotment.subject",
"foreignField" : "_id",
"as" : "allotment.subject"
}
},
{
"$lookup" : {
"from" : "teachers",
"localField" : "allotment.teacher",
"foreignField" : "_id",
"as" : "allotment.teacher"
}
},
{
"$addFields" : {
"allotment.subject" : {
"$arrayElemAt" : [
"$allotment.subject",
0.0
]
},
"allotment.teacher" : {
"$arrayElemAt" : [
"$allotment.teacher",
0.0
]
}
}
},
{
"$group" : {
"_id" : "$_id",
"isAssigned" : {
"$first" : "$isAssigned"
},
"name" : {
"$first" : "$name"
},
"section" : {
"$first" : "$section"
},
"allotment" : {
"$addToSet" : "$allotment"
}
}
}
]
)

Answer 3

Firstly you have to $unwind the allotment array and then apply $lookup for subject and then repeat same for teachers and finally apply $group to combine back it inside array. See below aggregate query that is have tried and its working for me.
Course.aggregate([
{"$match": matchQuery},
{
$unwind: '$allotment'
},
{
$lookup:{
"from": "subjects",
"localField": "allotment.subject",
"foreignField": "_id",
"as": "allotment.subject"
}
},
{
$unwind: '$allotment.subject'
},
{
"$lookup": {
"from": "teachers",
"localField": "allotment.teacher",
"foreignField": "_id",
"as": "allotment.teacher"
}
},
{
$unwind: '$allotment.teacher'
},
{
"$group" : {
"_id" : "$_id",
"isAssigned" : {
"$first" : "$isAssigned"
},
"name" : {
"$first" : "$name"
},
"section" : {
"$first" : "$section"
},
"allotment" : {
"$addToSet" : "$allotment"
}
}
}
])