Unable to filter out paths containing self edges using a cypher Match query.

[pushkarnagpal]

Trying the following query to get a path using a match query to filter out paths that contain self edges (node with edges to itself to avoid increased length of path:

match p = (base:party{node_id:'245793137123'})<-[trail:ownership*..]-(leaves:party) where none( rel in relationships(p) where startNode(rel) = endNode(rel) ) return count(p)

But this query is unable to filter out the data and I get the entire set.

[dwitry]

Hello @pushkarnagpal,

Are you sure you've provided a valid Cypher query?

It does not compile in Cypher for Gremlin and Neo4j gives error about multiple WHERE cases.

[pushkarnagpal]

Hello @pushkarnagpal,

Are you sure you've provided a valid Cypher query?

It does not compile in Cypher for Gremlin and Neo4j gives error about multiple WHERE cases.

Hi @dwitry I've updated the query now. (had accidentally written not instead of none in the query.

match p = (base:party{node_id:'245793137123'})<-[trail:ownership*..]-(leaves:party) where none( rel in relationships(p) where startNode(rel) = endNode(rel) ) return count(p)

Please look into the issue now. Thanks!

[dwitry]

Variable length path in loops is a complex case, which is tricky to implement in Gremlin to fully cover all the edge cases. I've invested a lot of time trying to properly implement it (1, 2) but the solution still not universal. Unfortunately can not provide any estimates when this will be improved.

[pushkarnagpal]

Variable length path in loops is a complex case, which is tricky to implement in Gremlin to fully cover all the edge cases. I've invested a lot of time trying to properly implement it (1, 2) but the solution still not universal. Unfortunately can not provide any estimates when this will be improved.

Ahh okay. I was working on a fraud detection case.and got stuck here. I saw this query conversion from the cypher in the gremlin-server.log file, and I see a .neq(' cypher.null') in the query and i assume this step is occuring at the none where clause. I think this is the root cause of the bug.

g.V().as('base')
	.hasLabel('party').has('node_id', eq('245793137123'))
	.repeat(__
				.inE('ownership')
				.as('trail')
				.aggregate('  cypher.path.edge.p')
				.outV()
		)
	.emit()
	.until(__
			.path()
			.from('base')
			.count(local)
			.is(gte(21))
		)
	.as('leaves')
	.hasLabel('party')
	.path()
	.from('base')
	.as('p')
	.where(__
			.not(__.V().hasLabel('party').outE('ownership').inV().as('  GENERATED4').where(__.select('  GENERATED4').where(eq('leaves'))))
		)
	.optional(__
		.select(all, 'trail').as('trail')
		)
	.select('p')
	.is(neq('  cypher.null'))
	.count()
	.project('count(p)')
	.by(__.identity())

Hope I could help somehow with this. Thanks!

[dwitry]

.neq(' cypher.null') is guard to handle cypher.null which represents null in Cypher for Gremlin.

We may call "root cause" the following snippet:

.repeat(__
            .inE('ownership')
            .as('trail')
            .aggregate('  cypher.path.edge.p')
            .outV()
    )
.emit()
.until(__
        .path()
        .from('base')
        .count(local)
        .is(gte(21))
    )

This is how variable length path <-[trail:ownership*..]- is translated. Basically, it is an imperative way to traverse relationships (21 is limit to avoid infinite loops). If there is a loop, the path will be traversed repeatedly (what happens in the query you've provided).

So we need to find a better implementation of this in Gremlin, but as I've said in the previous message this is a rather complicated task, as there are other edge cases that need to be considered, as well as performance.