Neo4j和cypher：如何匹配/创建路径的特定部分？

Question

I have pre-existing nodes a and b , and want to create a path

(a) <-[:FROM]- (c:Comp) -[:TO]-> (b)

Unless it already exists. create unique doesn't quite work here, because if we already have

(a) <-[:FROM]- (d:Comp) -[:TO]-> (e)

Then d will get reused as c , and only the relationship (c) -[:TO]-> (b) will get created. I want to create a new :Comp node in this case.

Whether or not the :Comp node was just created, I subsequently want to create unique a relationship from it.

I've come up with two possible solutions that I don't particularly like:

I think I could do it with optional relationships, something like

match (a) <-[r1?:FROM]- (c:Comp) -[r2?:TO]-> (b)
where r1 is null or r2 is null
create (a) <-[:FROM]- (d:Comp) -[:TO]-> (b)

But I'm not sure how I'd create the extra relationship if the :Comp node already exists.

Another option is to have properties on c uniquely identifying a and b , along the lines of

(a) <-[:FROM]- (c:Comp { from: a.uuid, to: b.uuid }) -[:TO]-> (b)

So that create unique won't start with a partial match. But having redundant data lying around is a smell.

Answer 1

I created what I think is your model in a console: http://console.neo4j.org/?id=l6q3q0

Perhaps try this:

START a=node:node_auto_index(name='a'), b=node:node_auto_index(name='b')
WHERE NOT (a<-[:FROM]-(:Comp)-[:TO]->b)
CREATE a<-[:FROM]-(c:Comp), c-[:TO]->b

Answer 2

I haven't tried yet, but it looks like MERGE can do this as of 2.0.0-rc1. http://docs.neo4j.org/chunked/milestone/query-merge.html

I'll want something like

MERGE (a) <-[:FROM]- (c:Comp) -[:TO]-> (b)
CREATE UNIQUE (c) -[:SUGGESTED_BY]-> (x:Algo)