Mongo Query: how to $lookup with DBRef - mongodb

Question

I have a trouble with $lookup with DBRef. I couldn't find the solution for below scenario anywhere. Someone please help me here?
Suppose the Collection A is
{
"_id" : ObjectId("582abcd85d2dfa67f44127e0"),
"status" : NumberInt(1),
"seq" : NumberInt(0) }
and Collection B:
{
"_id" : ObjectId("582abcd85d2dfa67f44127e1"),
"Name" : "from B Collection"
"bid" : DBRef("B", ObjectId("582abcd85d2dfa67f44127e0")) }
I have spent lot of time in aggregating above two collections. I am looking for the output as below.
{
"_id" : ObjectId("582abcd85d2dfa67f44127e0"),
"status" : NumberInt(1),
"seq" : NumberInt(0),
B: [
{
"_id" : ObjectId("582abcd85d2dfa67f44127e1"),
"Name" : "from B Collection"
}
]}
Please help me with the Mongo query to retrieve the result in the above format. Thanks in advance

Answer 1

Ideally you would be able to change the DBRef to a plain objectId or just string type. As noted in this post, it can be convoluted to use a DBRef in a lookup. The key is an $addFields stage with {$objectToArray: "$$ROOT.bid"} to get the DBRef value into a usable format.
You'll need to start the aggregation from collection B since that is where the reference is -- and that DBRef needs massaging before doing the lookup. Knowing that is the case, maybe the goal output shape might change; however, here is an aggregation that works to get you what you need:
db.getCollection('B').aggregate([
{$addFields: {fk: {$objectToArray: "$$ROOT.bid"}}},
{$lookup: {
from: 'A',
localField: 'fk.1.v',
foreignField: '_id',
as: 'A'
}},
// the below is transforming data into the format in the example
{$addFields: {'A.B': {_id: '$_id', Name: '$Name'}}},
{$unwind: '$A'},
{$replaceRoot: {newRoot: '$A'}}
])
You might need to do a groupBy if there are multiple B matches you need to group into an array.