How to traverse back to root vertex in gremlin in one single query

Question

I am using Gremlin in amazon-neptune. I have vertex user, country, order
I have edge 'lives_in' from user to country ,edge 'purchased' from user to order, edge 'delivered' from order to country

Goal : Find top most country which purchases most orders to foreign country rather than live_in country in descending order

gremlin> g.V().hasLabel("user").outE('purchased').inV().hasLabel("order"). 
......1> outE("delivered").inV().hasLabel("country").
......2> has('name').neq(outE('lives_in').inV().hasLabel("country").values()).
......3> groupCount().by(values)

I am not able to traverse back to root vertex from step neq(outE("lives_in"))
I am getting the same results after removing the last has step

gremlin> g.V().hasLabel("user").outE('purchased').inV().hasLabel("order").
......1> outE("delivered").inV().hasLabel("country")

This means my last has step is not executing.
Result sample - {v[country_GB]=38,v[country_NZ]=6,v[country_AU]=3}

stephen mallette stephen mallette · Accepted Answer · 2018-02-14T14:08:46

It's always helpful to include a small sample graph like this in your question:

g.addV('user').as('u1').
  addV('user').as('u2').
  addV('order').as('o1').
  addV('order').as('o2').
  addV('order').as('o3').
  addV('order').as('o4').
  addV('order').as('o5').
  addV('order').as('o6').
  addV('country').property('name','usa').as('usa').
  addV('country').property('name','candada').as('can').
  addV('country').property('name','mexico').as('mex').
  addE('lives_in').from('u1').to('usa').
  addE('lives_in').from('u2').to('mex').
  addE('purchased').from('u1').to('o1'). 
  addE('purchased').from('u1').to('o2').
  addE('purchased').from('u1').to('o3').
  addE('purchased').from('u1').to('o4').
  addE('purchased').from('u2').to('o5').
  addE('purchased').from('u2').to('o6').
  addE('delivered').from('o1').to('usa').
  addE('delivered').from('o2').to('mex').
  addE('delivered').from('o3').to('mex').
  addE('delivered').from('o4').to('can').
  addE('delivered').from('o5').to('mex').
  addE('delivered').from('o6').to('can').iterate()

Based on that, here's one way you might do this:

gremlin> g.V().hasLabel("user").as('u').
......1>   out('lives_in').hasLabel("country").as('c'). 
......2>   select('u').
......3>   out('purchased').hasLabel("order").
......4>   out("delivered").hasLabel("country").
......5>   where(neq('c')).
......6>   groupCount().
......7>     by('name')
==>[mexico:2,candada:2]

A few things to note:

Simplify inE().outV() and outE().inV() to just in() and out() respectively if you aren't doing anything with the edge.
At the line marked 1, the "lives_in" country vertex is labelled for later comparison against the "delivered" countries at line 5
The result excludes order labelled "o1" and the order labelled "o5" as both of those orders originated in the country to which they were shipped.

How to traverse back to root vertex in gremlin in one single query

1 Answers