CODEWITHSUNDEEP
neo4jcypher
altimit
altimit

Reputation: 421

return start and end nodes for a list of relationships in cypher

I want to get the start and end node for a list of relationships. In the example below, I start with a node and its related to the next node, and that next node to related to another node via the same relationship.

I tried the following cypher query, but it doesn't work. 

start n=node:index('name:"iris"')
match n-[r:childof*]->parent
return startNode(r),endNode(r)

Any ideas? I would think a foreach on r will do the trick but cant seem to make the could work.

Upvotes: 2

Views: 4136

Answers (1)

jjaderberg
jjaderberg

Reputation: 9952

As you don't say otherwise I'm assuming you are using current version (2.0) of Neo4j, and that "it doesn't work" means that startNode(r) complains about r being a collection, not a relationship. To run a function or evaluate an expression for every member of a collection use extract or reduce (foreach is for write operations)

RETURN EXTRACT(rel IN r | [startNode(rel),endNode(rel)]) as nodePairCollection

which for a path like a-[:REL]->b-[:REL]->c-[:REL]->d gives you a result like [[a,b],[b,c],[c,d]], or 

RETURN REDUCE(acc = [], rel IN r | acc + startNode(rel) + endNode(rel)) as flattenedNodeCollectionWithDulicates

which gives [a,b,b,c,c,d], or yet do some filtering on one of these.

That's how to explicitly get start node and end node from a relationship. But for your particular type of query above, where all the relationships in r have the same direction, you may be able to make do with just retuning the nodes on the path since they will already be correctly ordered

MATCH path=n-[r:childof*]->parent
RETURN nodes(path)

which gives [a,b,c,d], but you can only trust this as representing relationship direction when you have explicitly included that in your pattern.

Upvotes: 3

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