count of $lookup result mongodb - mongodb

Question

Using mongodb with NodeJS driver
I have 2 collections. One for department and other for students.
Sample data for Deparmtent.
{
"_id" : ObjectId("5a24d20590d3d12155f3094e"), "name" : "CSE", "hod" :
"abc",
"students" : [
ObjectId("5a2129172c3e542acb78c1f5"),
ObjectId("5a2129172c3e542acb78c1f7"),
ObjectId("5a2129172c3e542acb78c1f9"),
ObjectId("5a2129172c3e542acb78c1fb")
]
}
{
"_id" : ObjectId("5a24d20590d3d12155f3094f"),
"name" : "IT",
"hod" : "xyz", ,
"students" : [
ObjectId("5a2129172c3e542acb78c1f6"),
ObjectId("5a2129172c3e542acb78c1f8"),
ObjectId("5a2129172c3e542acb78c1fa"),
ObjectId("5a2129172c3e542acb78c1fc")
]
}
and sample data for Students
{
"_id" : ObjectId("5a2129172c3e542acb78c1f4"),
"rollNumber" : 11,
"name" : "Thor",
"branch" : ObjectId("5a24d20590d3d12155f3094e"),
}
Here is the query:
aggregate([
{$lookup:
{ from: "students",
localField: "_id",
foreignField: "branch",
as: "studentData"
}
}
])
Output of the query
[
{ _id: 5a24d20590d3d12155f3094e,
name: 'CSE',
hod: 'abc',
studentData: [ [Object], [Object], [Object], [Object], [Object] ]
},
{ _id: 5a24d20590d3d12155f3094f,
name: 'IT',
hod: 'xyz',
studentData: [ [Object], [Object], [Object], [Object] ] }
]
I have two questions
How to print complete student data.
How to get the count of studentData for each Department

Answer 1

To get complete student data you can do something like
db.student.aggregate(
[
{
$lookup:
{
from:"department",
localField:"branch",
foreignField:"_id",
as:"branch"
}
}
]
)
This will give you something like this:
{
"_id" : 1.0,
"rollNumber" : 110.0,
"name" : "Thor",
"branch" : [
{
"_id" : 1.0,
"name" : "CSE",
"hod" : "abc"
}
]
}
To get the count of studentData for each Department
db.getCollection('student').aggregate(
[
{
$lookup:
{
from:"department",
localField:"branch",
foreignField:"_id",
as:"branch"
}
},
{
$group:
{
_id:"$branch",
"numOfStudent":{$sum:1},
"listOfStudents":{$push:"$name"}
}
}
]
)
This will give you something like this:
{
"_id" : [
{
"_id" : 2.0,
"name" : "IT",
"hod" : "xyz"
}
],
"numOfStudent" : 1.0,
"listOfStudents" : [
"Ironman2"
]
}
{
"_id" : [
{
"_id" : 1.0,
"name" : "CSE",
"hod" : "abc"
}
],
"numOfStudent" : 3.0,
"listOfStudents" : [
"Thor",
"Ironman",
"Ironman3"
]
}
You can change $push:$name to $push:$_id If you want to store the Ids of students and not their names.
EDIT
Get similar result using "Departments" collection:
db.department.aggregate([
{
$lookup:
{
from:"student",
localField:"students",
foreignField:"_id",
as:"studentsDetails"
}
},
{
$project:{
_id:0,
name:"$name",
hod:"$hod",
numOfStudents:{$size:"$studentsDetails"},
students:"$studentsDetails"
}
}
])
This will give you something like this:
{
"name" : "CSE",
"hod" : "abc",
"numOfStudents" : 2,
"students" : [
{
"_id" : 1.0,
"rollNumber" : 110.0,
"name" : "Thor",
"branch" : 1.0
},
{
"_id" : 3.0,
"rollNumber" : 111.0,
"name" : "Ironman2",
"branch" : 2.0
}
]
}

Answer 2

Your $lookup is completely fine. To add a count of students to your output data, you can use $addFields.
Add this to your aggregation:
{ $addFields: {studentCount: {$size: "$studentData"}}}
}
So, your query should be something like this:
aggregate([
{$lookup:
{ from: "students",
localField: "_id",
foreignField: "branch", as: "studentData"
}
}, { $addFields: {studentCount: {$size: "$studentData"}}
}
])