Gremin mergeV() Upsert: knowing whether the vertex was added or not(side-effect free)

In CosmosDB Graph : "upsert" query pattern, there was an awesome discussion of using fold()/coalesce()/unfold() style pattern for upserting vertex, and knowing whether the vertex has been added or obtained:

g.V().has('event','id','1').
  fold().
  coalesce(unfold().
           project('vertex','exists').
             by(identity()).
             by(constant(true)),
           addV('event').property('id','1').
           project('vertex','exists').
             by(identity()).
             by(constant(false)))

The post also made the update that there was a more advanced pattern using mergeV(). I wonder if we can "translate" the above to the more advanced mergeV() pattern?

Any help/insight is much appreciated!

The new mergeV() pattern seems to expect a list as the inputs, so i am not sure how to map that from the original queries.

You can use onMatch and onCreate to know what happened during the mergeV search. Here is one way to build on that to generate a result similar to the one described in the question.

g.mergeV([T.id:'1']).
    option(Merge.onMatch,sideEffect(constant(true).store('found')).constant([:])).
    option(Merge.onCreate,sideEffect(constant(false).store('found')).constant([:])).
  project('id','status').
    by(id()).
    by(select('found').unfold())

As the vertex with an ID of '1' does exist, when run, this is what the query returns:

{'id': '1', 'status': True}

Strictly speaking in this case you don't need the sideEffect wrapper, but I find it cleaner as it isolates that work from the rest of the onMatch steps.