Using Project + valueMap in combination with where

Question

I have the following graph: https://gremlify.com/vxkibjtoz3b if u can't open the link the graph is lloking the following:

You can see there 2 queries. The first one: g.V().out("r").as("a").V().out("rl_pre").out("rl").as("b").where("a", eq("b")).by("attr").select("a", "b")

gives me all vertices which share the same "attr" value. In the second query: g.V().out("r").as("a").V().hasLabel("start").project("lower_pre", "lower"). by(__.out("rl_pre").valueMap("lower_pre_prop")) by(__.out("rl_pre").out("rl").as("b").where("a", eq("b")).by("attr").valueMap("attr"))

I tried to combine the first query with "project" and "valueMap" to get some values of former attributes. Unfortunately the second query isn't working. Does anyone of know who to get properties of former Vertices + the properties in the graph traversal end vertex in combination with the "where" clause?

Expected output

I expect to get all vertex pairs ("upper and lower) which share the same property value for ("attr") + I want to have the "start_attribute" value from the start vertex

Kind regards Stefan

The first query is going to potentially fan out unexpectedly and be expensive as V (all vertices) will be applied to the results of g.V().out('r'). You may want to fold first to flatten all of the traversals. Can you sat a bit more about the exact output you are looking to produce? — Kelvin Lawrence
– Kelvin Lawrence, Commented Mar 14, 2023 at 14:51
Thx for your reply... I edit my article and added the section: Expected output. Hope thie helps :) — Ste fan
– Ste fan, Commented Mar 14, 2023 at 15:31

Kelvin Lawrence · Accepted Answer · 2023-03-14 21:11:15Z

1

Taking the Gremlify data and pretty printing it a little gives us:

g.addV('wrong_node').as('1').property(single, 'attr', 0).
  addV('start').as('2').
  addV('upper').as('3').property(single, 'attr', 1).
  addV('lower').as('4').property(single, 'attr', 1).
  addV('lower_pre').as('5').property(single, 'lower_pre_prop', 666).
  addV('lower_wrong').as('6').property(single, 'attr', 2).
  addV('upper_2').as('7').property(single, 'attr', 1).
  addE('r').from('2').to('7').
  addE('r').from('2').to('3').
  addE('r').from('2').to('1').
  addE('rl_pre').from('2').to('5').
  addE('rl').from('5').to('4').
  addE('rl').from('5').to('6')

Using that sample graph, the query can be reduced to:

gremlin> g.V().hasLabel('start').as('s').
......1>   out('r').as('a').
......2>   V().hasLabel('start').
......3>   out('rl_pre').
......4>   out('rl').
......5>   where(eq('a')).
......6>     by('attr').as('b').
......7>   select('s','a','b').
......8>     by().
......9>     by(valueMap(true,'attr')).
.....10>     by(valueMap(true,'attr'))

which produces (I did not see a "start_attribute" property so left that part out)

==>[s:v[2],a:[id:11,label:upper_2,attr:[1]],b:[id:5,label:lower,attr:[1]]]
==>[s:v[2],a:[id:3,label:upper,attr:[1]],b:[id:5,label:lower,attr:[1]]]

The valueMap step can be replaced by elementMap in this case which gives a slightly simpler query and more readable output.

gremlin> g.V().hasLabel('start').as('s').
......1>   out('r').as('a').
......2>   V().hasLabel('start').
......3>   out('rl_pre').
......4>   out('rl').
......5>   where(eq('a')).
......6>     by('attr').as('b').
......7>   select('s','a','b').
......8>     by().
......9>     by(elementMap('attr')).
.....10>     by(elementMap('attr'))  

==>[s:v[2],a:[id:11,label:upper_2,attr:1],b:[id:5,label:lower,attr:1]]
==>[s:v[2],a:[id:3,label:upper,attr:1],b:[id:5,label:lower,attr:1]]

edited Mar 14, 2023 at 21:11

answered Mar 14, 2023 at 20:15

Kelvin Lawrence

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2 Comments

Ste fan Over a year ago

Super cool and detailed answer. Thank you very much :) Did u like the idea of sharing the graph via gremlify?

Kelvin Lawrence Over a year ago

Gremlify is helpful yes, but I also like to have the graph creation steps in the post itself in case one day the graph in Gremlify is deleted etc.

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