Summary

Projection on document ID alone creates invalid projection.

C# Driver Version: 2.19.1, 2.19.2

MongoDB Version: 4.0, 4.2
MongoDB Topology: Using the Mongo driver for Azure; sharded cluster I think?

How to Reproduce

Attached is a small console application to demonstrate the error. I tested with both a string ID and an ObjectId ID, but both fail: strings are null, ObjectIDs are empty.

The projection is pretty simple, in the test case, it's just:

collection.Find(_ => true).Project(x => x.Id)

(Finding all documents just for the small test I have.)

However, the projection it generates in v2.19+ is:

find({ }, { "_v" : "$_id", "_id" : 0 })

In prior versions (we were using 2.14), this code worked, and the generated projection is what I'd have expected:

find({ }, { "_id" : 1 })