Coalesce with Constant

Hi, I have a problem with projecting connected vertexes. I would like to receive Tuple that contains information regarding ‘Team’ and ‘Stadium’ so this is the query that i’m currently working on :

Gremlin
  .V<Team>()
  .Project(t => t
    .ToTuple()
    .By(t => t.Cast<Team>())
    .By(__ =>
    {
	return __.Cast<Stadium>().In<PlaysOn>().OfType<Stadium>()
    }));

This works fine when there is data for both Team and Stadium. Problem is when there is no corresponding data for Stadium. I am receiving this information:

Gremlin Query Execution Error: Project By: Next: The provided traverser of key “Item2” maps to nothing.

I thought that in case when there is no corresponding ‘Stadium’, constant of null could help, but i am not able build write ‘Coalesce’ with ‘Constant’. Here is how i would imagine it (code not compiling) :

Gremlin
  .V<Team>()
  .Project(t => t
    .ToTuple()
    .By(t => t.Cast<Team>())
    .By(__ =>
    {
	return __.In<PlaysOn>().OfType<Stadium>().Fold().Coalesce(
	  _ => _.Unfold(),
	  _ => _.Constant<Stadium>(null));
    }));

Here is the query that works on gremlin

g.V()
  .hasLabel('Team').as('T')
  .project('team', 'stadium')
    .by(select('T'))
    .by(__.in('PlaysOn').has('label', 'Stadium').fold().coalesce(
	unfold(), constant('No Stadium')))

I tried to use Constant in different ways but i was not able to make it work. Could you also give some simple example how this should look like?

[danielcweber]

Slip a ForceValue() in before the casts. Currently, one is a IValue… and the other is a IVertex…ForceValue will make the interfaces equal.

Actually, why even do the whole Coalesce-thingy. If you change the view on the domain such that teams may have 0-n stadiums, a single Fold() will do.

[BlaszczykB]

Thank you, both solutions are working