Group by path

[Robert Dale]

I would like to group some paths by their destination. However, it appears that only the last path is available in the resulting group.

So for the example graph below, I would like to get all paths taken to destinations a or b.

graph = TinkerGraph.open()

g = graph.traversal()

g.addV('start').property('name', 'start').as('start').

addV('level1').property('name', '1').as('1').

addV('level1').property('name', '2').as('2').

addV('level1').property('name', '3').as('3').

addV('level1').property('name', '4').as('4').

addV('level2').property('name', 'a').as('a').

addV('level2').property('name', 'b').as('b').

addE('next').from('start').to('1').

addE('next').from('start').to('2').

addE('next').from('start').to('3').

addE('next').from('start').to('4').

addE('next').from('1').to('a').

addE('next').from('2').to('a').

addE('next').from('3').to('b').

addE('next').from('4').to('b')

Here we can see that there are 2 paths for each destination.

g.V().hasLabel('start').out().as('level1').out().as('level2').path().by(values('name'))

==>[start,1,a]

==>[start,2,a]

==>[start,3,b]

==>[start,4,b]

When grouped, only the last path of each group remains:

g.V().hasLabel('start').out().as('level1').out().as('level2').group().by('name').by(path().by(values('name')))

==>[a:[start,2,a],b:[start,4,b]]

I know they are there because...

g.V().hasLabel('start').out().as('level1').out().as('level2').group().by('name').by(path().unfold().values('name').fold())

==>[a:[start,1,a,start,2,a],b:[start,3,b,start,4,b]]

In the end, I got it to work by putting the path in the main traversal and grouping on the last index in the path.

g.V().hasLabel('start').out().as('level1').out().as('level2').path().from('level1').to('level2').by('name').as('path').group().by(tail(local))

==>[a:[[1,a],[2,a]],b:[[3,b],[4,b]]]

Good results but let's get rid of the duplicate destinations 'a','b' in the values part. I'll try to limit what's in the path.

g.V().hasLabel('start').out().as('level1').out().as('level2').path().from('level1').to('level2').by('name').as('path').group().by(tail(local)).by(limit(local, 1))

==>[a:2,b:4]

Doh! I tried a few thing but couldn't get this one to work.  Back to square 1!

So, the question is:  Why don't these things work the way I expect it to?  :-)

[Robert Dale]

I think I get it now.  If a traversal is provided, then it's left to the traversal to provide the fold().

These now work as expected.

g.V().hasLabel('start').out().as('level1').out().as('level2').group().by('name').by(path().from('level1').to('level2').by(values('name')).fold())

==>[a:[[1,a],[2,a]],b:[[3,b],[4,b]]]

g.V().hasLabel('start').out().as('level1').out().as('level2').path().from('level1').to('level2').by('name').as('path').group().by(tail(local)).by(limit(local, 1).fold())

==>[a:[1,2],b:[3,4]]

Yay.