Gremlin/Tinkerpop - is there a way to add metadata to a union step so I know which query the resulting traversal came from?

This is a little strange, but I have a situation where it'd be beneficial for me to know which traversal an element came from.

For a simple example, something like this:

.union(
   select('parent').out('contains'),   //traversal 1
   select('parent2').out('contains')   //traversal 2
)
.dedup()
.project('id','traversal')
.by(id())
.by( //any way to determine which traversal it came from? or if it was in both? )

Edit: One thing I found is that I can use Map with Group/By to get partly there:

.union(
   select('parent').out('contains')
   .map(group().by(identity()).by(constant('t1'))),  
   select('parent2').out('contains')   
   .map(group().by(identity()).by(constant('t2'))), 
)
.dedup() //Dedup isn't gonna work here because each hashmap will be different.
.project('id','traversal')
.by( //here I can't figure out how to read a value from the hashmap inline )

The above query without the project/by piece returns this:

[{v[199272505353083909]: 't1'}, {v[199272515180338177]: 't2'}]

Or is there a better way to do this?

Thanks!

Solution

One simple approach might be to just fold the results. If you get back an empty list you will know you did not find any on that "branch":

g.V('44').
  union(out('route').fold().as('a').project('res','branch').by().by(constant('b1')),
        out('none').fold().as('b').project('res','branch').by().by(constant('b2')))

which yields

{'res': [v[8], v[13], v[20], v[31]], 'branch': 'b1'}
{'res': [], 'branch': 'b2'}

UPDATED after discussion in comments to include an alternative approach that uses nested union steps to avoid the project step inside the union. I still think I prefer the project approach unless the performance when measured is not good.

g.V('44').
  union(local(union(out('route').fold(),constant('b1')).fold()),
        local(union(out('none').fold(),constant('b2')).fold()))

which yields

[[v[8], v[13], v[20], v[31]], 'b1']
[[], 'b2']