I was not in favour of using the solution in presentation as it involves making multiple queries for making an upsert. Was wondering if there is a native way to do that in a single statement without encountering race conditions.
I understand changes to a document are atomic in general. But was worried about ending up with two simultaneous $push resulting in two duplicate subdocuments in array.
The stack overflow answer linked also does it in two steps (first steps removes the record from array if it exists and second step inserts). But still feel it is prone to race conditions.
The second solution does something similar but uses the aggregation pipeline. Do you know what lock is acquired by the read statement in a pipeline? i.e, if two servers have sent the same query to mongodb, does mongodb serialize the full pipeline or just the write parts of the pipeline?