Description

The $ positional operator behaves ambiguously when there are multiple matching arrays. The work tracked on SERVER-18500 has not seen any progress, but we see this ambiguity causing issues with some users in SERVER-54687 and SERVER-53896.

https://docs.mongodb.com/manual/reference/operator/update/positional/

We should clarify we currently don't guarantee which array we get the position from when using the positional operator when there are multiple array matches.
Example:

doc = { a: [1, 2], z: [1, 2], b: [1, 2] }
db.coll.insert(doc);
db.coll.update({ a: 1, z: 2, b: 1 }, { $set: { "b.$": 3 } });

There is ambiguity around which index of the matched arrays (0 or 1) should we update b.
The current behavior returns:

{ "a" : [ 1, 2 ], "z" : [ 1, 2 ], "b" : [ 1, 3 ] }

where we update b using the index matched in z (index 1).

We may be able to guide users to consider $[<identifier>] if they need to make changes to an array based on a specified position. An example of favoring $[<identifier>] over $ can be found in my closing comment on SERVER-54687

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)