How can I compare two fields in diffrent two collections in mongodb? - mongodb

Question

I am beginner in the MongoDB.
Right now, I am making one query by using mongo. Please look this and let me know is it possible? If it is possible, how can I do?
collection:students
[{id:a, name:a-name}, {id:b, name:b-name}, {id:c, name:c-name}]
collection:school
[{
name:schoolA,
students:[a,b,c]
}]
collection:room
[{
name:roomA,
students:[c,a]
}]
Expected result for roomA
{
name:roomA,
students:[
{id:a name:a-name isRoom:YES},
{id:b name:b-name isRoom:NO},
{id:c name:c-name isRoom:YES}
]
}

Answer 1

Not sure about the isRoom property, but to perform a join across collections, you'd have two basic options:
code it yourself, with multiple queries
use the aggregation pipeline with $lookup operator
As a quick example of $lookup, you can take a given room, unwind its students array (meaning separate out each student element into its own entity), and then look up the corresponding student id in the student collection.
Assuming a slight tweak to your room collection document:
[{
name:"roomA",
students:[ {studentId: "c"}, {studentId: "a"}]
}]
Something like:
db.room.aggregate([
{
$unwind: "$students"
},
{
$lookup:
{
from: "students",
localField: "studentid",
foreignField: "id",
as: "classroomStudents"
}
},
{
$project:
{ _id: 0, name : 1 , classroomStudents : 1 }
}
])
That would yield something like:
{
name:"roomA",
classroomStudents: [
{id:"a", name:"a-name"},
{id:"c", name:"c-name"}
]
}
Disclaimer: I haven't actually run this aggregation, so there may be a few slight issues. Just trying to give you an idea of how you'd go about solving this with $lookup.
More info on $lookup is here.