simplePath() breaks query

Question

When ading simplePath() inside a match(), my query no longer returns results.

The query attempts to find any event (e.g. "graph database conference") that somehow involves three specific people.

"alice" attended the school that hosted the event.
"bob" was a hot dog vendor at the event.
"marko" provided security for the event.

I'm using match() to find where the three people converge. If there's a better way, please suggest it. Thanks! Just starting to learn gremlin.

Ascii art:

            alice --[enrolled-in]-> gremlin 101 --[offered-by]-> graph db school --[hosted]--------------
                                                                                                        |
                                                                                                        v
bob --[works-for]-> hot dogs r awesome --[subcontractor-of]-> best event planner --[planned]----> graph conference
                                                                                                        ^
                                                                                                        |
                                            marko --[works-for]-> super security --[secured]-------------

Query that works:

g.V().match(
  __.as('alice').hasLabel('person').has('name', 'alice').repeat(__.out()).until(__.hasLabel('event')).as('event'),
  __.as('event').repeat(__.in()).until(__.hasLabel('person').has('name', 'bob')).as('bob'),
  __.as('event').repeat(__.in()).until(__.hasLabel('person').has('name', 'marko')).as('marko')).
  path()

==>[v[0],v[0],v[2],v[5],v[21],v[21],v[13],v[10],v[8],v[21],v[18],v[16],[bob:v[8],alice:v[0],event:v[21],marko:v[16]]]

Note that some vertices appear more than once (and we haven't even added any cycles yet!)

When I add .simplePath() to any of the repeat()s, the query returns nothing. For example, inside the first repeat()

g.V().match(
  __.as('alice').hasLabel('person').has('name', 'alice').repeat(__.out().simplePath()).until(__.hasLabel('event')).as('event'),
  __.as('event').repeat(__.in()).until(__.hasLabel('person').has('name', 'bob')).as('bob'),
  __.as('event').repeat(__.in()).until(__.hasLabel('person').has('name', 'marko')).as('marko')).
  path()

gremlin-console:

alice = g.addV('person').property('name', 'alice').next()
gremlin101 = g.addV('course').property('name', 'gremlin 101').next()
g.addE('enrolled-in').from(alice).to(gremlin101)

school = g.addV('school').property('name', 'graph db school').next()
g.addE('offered-by').from(gremlin101).to(school)

bob = g.addV('person').property('name', 'bob').next()
hotDogs = g.addV('business').property('name', 'hot dogs r awesome').next()
g.addE('works-for').from(bob).to(hotDogs)

eventPlanner = g.addV('business').property('name', 'best event planner').next()
g.addE('subcontractor-of').from(hotDogs).to(eventPlanner)

marko = g.addV('person').property('name', 'marko').next()
security = g.addV('business').property('name', 'super security').next()
g.addE('works-for').from(marko).to(security)

event = g.addV('event').property('name', 'graph conference').next()
g.addE('hosted').from(school).to(event)
g.addE('secured').from(security).to(event)
g.addE('planned').from(eventPlanner).to(event)

score 1 · Accepted Answer · answered Oct 29 '18 at 19:05

match() and simplePath() will almost certainly never work well together. If match() produces a simple path, then match() was really pointless. To find all the matching events, you would do something like this:

gremlin> g.V().has("person", "name", within("alice","bob","marko")).as("p").
......1>   repeat(out().simplePath()).
......2>     until(hasLabel("event")).
......3>   group().
......4>     by("name").
......5>     by(group().
......6>          by(select("p").by("name")).
......7>          by(path().by("name").fold())).unfold().
......8>   filter(select(values).count(local).is(3)).
......9>   select(keys)
==>graph conference

And if you're also interested in the paths from each person to the event:

gremlin> g.V().has("person", "name", within("alice","bob","marko")).as("p").
......1>   repeat(out().simplePath()).
......2>     until(hasLabel("event")).
......3>   group().
......4>     by("name").
......5>     by(group().
......6>          by(select("p").by("name")).
......7>          by(path().by("name").fold())).unfold().
......8>   filter(select(values).count(local).is(3)).
......9>   select(values).unfold().
.....10>   select(values)
==>[[bob,hot dogs r awesome,best event planner,graph conference]]
==>[[alice,gremlin 101,graph db school,graph conference]]
==>[[marko,super security,graph conference]]

Note that each row is an array of paths; that's because - in theory - every person could be connected to a specific event in more than just one way. If you're only interested in any connection between a person and an event, you can remove the fold() step from the nested group() step.

gremlin> g.V().has("person", "name", within("alice","bob","marko")).as("p").
......1>   repeat(out().simplePath()).
......2>     until(hasLabel("event")).
......3>   group().
......4>     by("name").
......5>     by(group().
......6>          by(select("p").by("name")).
......7>          by(path().by("name"))).unfold().
......8>   filter(select(values).count(local).is(3)).
......9>   select(values).unfold().
.....10>   select(values)
==>[bob,hot dogs r awesome,best event planner,graph conference]
==>[alice,gremlin 101,graph db school,graph conference]
==>[marko,super security,graph conference]

As a gremlin novice, I saw these as a wall of text. But I once peeled layers off and added them back, one at a time, the queries made sense. The `count(local).is(3)` threw me but now I see that requires all three people to have attended the event. This was really helpful. — deridex, Oct 30 '18 at 03:48

simplePath() breaks query

1 Answers1