.match() in Gremlin-Python

[Happy Feet]

I have a (gremlin 3.46, conf/gremlin-server-neo4j.yaml) db which I interact with via gremlinpython

The following queries both return results:

g.V(0).as_('a').out().match(as_('b').has("groupId",1)).select('b').next()

g.V(0).as_('a').out().match(as_('b').has("groupId",2)).select('b').next()

but I am unable to get the following query to work:

g.V(0).as_('a').out().match(as_('b').has("groupId",1), as_('c').has("groupId",2)).select('b', 'c').next()

I have tried wrapping the arguments to match in a tuple or a list but neither seems to work. 

For now, I have resorted to 'rewinding' back via select('a') to fetch multiple elements in a match, but this is ugly and probably suboptimal. 

[Stephen Mallette]

You match() with two has() on "group" would require the vertex to satisfy both of those filters and so it therefore returns no result. I simulated your traversal with the modern graph to demonstrate:

gremlin> g.V(1).as('a').out().
......1>   match(__.as('b').hasLabel('person')).
......2>   select('b')
==>v[2]
==>v[4]
gremlin> g.V(1).as('a').out().
......1>   match(__.as('c').hasLabel('software')).
......2>   select('c')
==>v[3]
gremlin> g.V(1).as('a').out().
......1>   match(__.as('b').hasLabel('person'),
......2>         __.as('c').hasLabel('software')).
......3>   select('b','c')

gremlin>

I'm not clear what result you want to achieve so I can't suggest other options. If you have a sample graph and provide some clarification on what you want to achieve, perhaps I could help further.

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[Happy Feet]

Hi Stephen,

I think I might have misunderstood the use of match. The query that I wanted to shorten with match() is:

g.V(0).as_('a').out()

         .has("groupId",1).as_('b')

         .select('a').out().has("groupId",2).as_('c')

         .select('b', 'c').next()

which selects all pairs of nodes (b,c) where b and c are in v(0).out() and have groupIds 1 and 2, respectively.

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[Stephen Mallette]

Maybe I'm oversimplifying but isn't this a case for or() which perhaps would be better written using within():

g.V().out().
  or(has("groupId",1),
     has("groupId",2))

g.V().out().has('groupId',within(1,2))

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[Happy Feet]

Yes, or() or within() work for that example.

The fault for oversimplifying is mine: more generally I wanted the following pattern (with potentially more than just b and c):

g.V().(some initial traversal).as_('a').(some traversal from a).as('b')

.select('a').(another traversal from a).as('c')

.(some traversal that expresses a relation between b and c).

.select('a', 'b', 'c')

I thought match() would allow me to express something like this more efficiently when the number of selected nodes/relations between them balloons.

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[Stephen Mallette]

>  I thought match() would allow me to express something like this more efficiently when the number of selected nodes/relations between them balloons.

I suppose it might, but I think we're talking in really general terms here so it's hard to say exactly what you need. I wouldn't go for a match() if my traversal could be more easily expressed. Personally, I don't find myself using match() all that often. In addition, more often than not, I tend to find that:

g.V().as('a').out().as('b').....as('c')....as('z').select('a',b','c',...'z')

patterns can be re-written for better performance and improved readability, so if you have a ballooning select() situation I'd wonder if there is a better way to accomplish what you're trying to do.

 

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