MongoDB 3.4 MongoDBRef dereferencing with .NET MongoDB Driver - mongodb

Question

I have following collections:
Collection A
{
"_id" : ObjectId("5aaa3b170e26ed1eba223ba9"),
"name" : "A1",
"ref" : {
"$ref" : "B",
"$id" : ObjectId("5aaa33740e26ed1eba223ba1")
}
}
{
"_id" : ObjectId("5aaa3b170e26ed1eba223baa"),
"name" : "A2",
"ref" : {
"$ref" : "C",
"$id" : ObjectId("5aaa33740e26ed1eba223ba2")
}
}
Collection B
{
"_id" : ObjectId("5aaa33740e26ed1eba223ba1"),
"name" : "B1"
}
...
Collection C
{
"_id" : ObjectId("5aaa33740e26ed1eba223ba2"),
"name" : "C1"
}
...
It is posable to get folowing result?
{
"_id" : ObjectId("5aaa3b170e26ed1eba223ba9"),
"name" : "A1",
"result" : [
{
"name" : "B1"
}
]
}
{
"_id" : ObjectId("5aaa3b170e26ed1eba223baa"),
"name" : "A2",
"result" : [
{
"name" : "C1"
}
]
}
I tried it with $Project and §Lookups, unfortunately withou success.
Hire is example:
db.A.aggregate([
{$project: {
name : 1,
refId: {$arrayElemAt: [{$objectToArray:"$ref"},1]},
refCol: {$arrayElemAt: [{$objectToArray:"$ref"},0]},
}
},
{$lookup : {
from : "refCol.v",
localField : "refId.v",
foreignField : "_id",
as : "result"
}
},
{$project : {"result._id" : 0, refId : 0, refCol : 0}}
])
In this example I can't reference the "refCol.v" field in the $lookup function.
Have someone a tip or a better solution for me?

Answer 1

You can achieve this by populate() method:
const collectionA = require('../models/collectionA');
collectionA.find({}).populate({
path: 'collectionB',
select: {
'name': 1,
'_id': 0
}
})
}).then((data) => {
if (data) {
res.send(data);
}
}).catch((err) => {
res.send(err);
})