Currently when updating an array with subdocuments according to a specific key, i.e. given array

[{id: 1, val: 'a'}, {id: 2, val: 'a'}, {id: 3, val: 'b'}]

, if we want to update ids 1 and 2 to new values, we would need to issue multiple update statements (which results in a blown oplog).

Adding an optional argument to addToSet to consider only specific fields would allow updating multiple subdocuments in a single call by only specifying the unique field.

This would both improve performance and reduce oplog size, and it seems like a simple addition.