How to push all values in single array in mongodb - mongodb

Question

Colleges
/* 1 createdAt:5/9/2019, 7:00:04 PM*/
{
"_id" : ObjectId("5cd42b5c65b41027845938ae"),
"clgID" : "100",
"name" : "Anna University"
},
/* 2 createdAt:5/9/2019, 7:00:04 PM*/
{
"_id" : ObjectId("5cd42b5c65b41027845938ad"),
"clgID" : "200",
"name" : "National"
}
Subjects:
/* 1 createdAt:5/9/2019, 7:03:24 PM*/
{
"_id" : ObjectId("5cd42c2465b41027845938b0"),
"name" : "Hindi",
"members" : {
"student" : [
"123"
]
},
"college" : {
"collegeID" : "100"
}
},
/* 2 createdAt:5/9/2019, 7:03:24 PM*/
{
"_id" : ObjectId("5cd42c2465b41027845938af"),
"name" : "English",
"members" : {
"student" : [
"456",
"789"
]
},
"college" : {
"collegeID" : "100"
}
}
Here i am having two collection and i want to join Colleges table is clgID and Subjects table iscollege.collegeID , then i want to take members.student values and push into single array based on college.collegeID.
My Expected Output
{
"GroupDetails" : [ ],
"clgName" : "National"
},
{
"GroupDetails" : [
"123",
"456",
"789"
],
"clgName" : "Anna University"
}
My Code
db.Colleges.aggregate([
{ $match : { "clgID" : { $in : ["100", "200"] } } },
{ $lookup: { from: "Subjects", localField: "clgID", foreignField: "college.collegeID", as: "GroupDetails" } },
//{ $unwind: "$GroupDetails" },
{ $project: { '_id' : false, 'clgName' : '$name', 'GroupDetails.members.student' : true } }
])
I am getting like this
/* 1 */
{
"GroupDetails" : [ ],
"clgName" : "National"
},
/* 2 */
{
"GroupDetails" : [
{
"members" : {
"student" : [
"456"
]
}
},
{
"members" : {
"student" : [
"123"
]
}
}
],
"clgName" : "Anna University"
}

Answer 1

You can use below aggregation with mongodb 3.6 and above
db.Colleges.aggregate([
{ "$match": { "clgID": { "$in": ["100", "200"] } } },
{ "$lookup": {
"from": "Subjects",
"let": { "clgId": "$clgID" },
"pipeline": [
{ "$match": { "$expr": { "$eq": ["$$clgId", "$college.collegeID"] } } },
{ "$group": {
"_id": "$college.collegeID",
"groupDetails": { "$push": "$members.student" }
}},
{ "$project": {
"groupDetails": {
"$reduce": {
"input": "$groupDetails",
"initialValue": [],
"in": { "$concatArrays": ["$$this", "$$value"] }
}
}
}}
],
"as": "clg"
}},
{ "$unwind": { "path": "$clg", "preserveNullAndEmptyArrays": true } },
{ "$project": {
"clgName": "$name",
"groupDetails": { "$ifNull": ["$clg.groupDetails", []] }
}}
])
MongoPlayground
Or with the mongodb 3.4 and below
db.Colleges.aggregate([
{ "$match": { "clgID": { "$in": ["100", "200"] }}},
{ "$lookup": {
"from": "Subjects",
"localField": "clgID",
"foreignField": "college.collegeID",
"as": "clg"
}},
{ "$unwind": { "path": "$clg", "preserveNullAndEmptyArrays": true }},
{ "$group": {
"_id": { "clgId": "$clg.college.collegeID", "_id": "$_id" },
"groupDetails": { "$push": "$clg.members.student" },
"clgName": { "$first": "$name" }
}},
{ "$project": {
"_id": "$_id._id",
"clgName": 1,
"groupDetails": {
"$reduce": {
"input": "$groupDetails",
"initialValue": [],
"in": { "$concatArrays": ["$$this", "$$value"] }
}
}
}}
])
MongoPlayground