Hi,

I'm trying to join two collections. My first collection has a string field, which contains id of a document in the second collection. Like this:

coll A:

{ bCollectionDocId: "231nkren32fko45"}

coll B:

{ _id: { $oid: "231nkren32fko45" } }

I can't come up with the right $lookup stage when local field is just string (bCollectionDocId) and foreign field is nested structure.

Perhaps, it's not possible at all?

Hi Pádraig,

It says: 'FieldPath field names may not start with '$'.

I think, that's impossible: https://jira.mongodb.org/browse/SERVER-14466

Well, I guess, I'll just then insert full {_id {$oid}} structure as a foreign key to the A collection. Like this:

coll A:

{ bCollectionDocId: {_id: { $oid: "231nkren32fko45" }} }

coll B:

{ _id: { $oid: "231nkren32fko45" } }

localField: "bCollectionDocId._id",

foreignField: "_id",

It will take a couple of hours to refactor the code in my project, but at least this solution seems to work.