Gremlin - select vertices based on value found in traversal

Question

I want to use the id of a vertex that has prop_a=x to find other vertices which have this value in some other property.
Something like:

g.V().sideEffect(has('prop_a','x').id().as('val')).has('prop_b',__.select('val')).count()

But the above doesn't return the right result.
In addition, I need it to run efficiently on AWS Neptun.

Answer 1

Queries to create the data:

gremlin> g.addV().property(id,1).property('prop_a','x')
==>v[1]
gremlin> g.addV().property(id,2).property('prop_b',1)
==>v[2]

Query to get the desired data:

gremlin> g.V().
......1>   has('prop_a', 'x').
......2>   id().as('val').
......3>   V().as('b').
......4>   values('prop_b').
......5>   where(eq('val')).
......6>   select('b')
==>v[2]