Creating relationship conditionally with cypher (neo4j)

3
votes

I am attempting to create a linked list with neo4j, and have a root node with no relationships. Here is the pseudo cypher I am trying to create, but I am not sure how, or even if it is possible:

START root=node(1), item=node(2)
MATCH root-[old?:LINK]->last
WHERE old IS NOT NULL
CREATE root-[:LINK]->item-[:LINK]->last
DELETE old
WHERE old IS NULL
CREATE root-[:LINK]->item

Basically I am trying to insert a node into the list if the list exists, and simply create the first list item otherwise. Obviously you cannot do multiple WHEREs like I have done above. Any ideas how I can achieve this desired functionality with a cypher?

The docs solve the problem by first creating a recurrent :LINK relationship on the root node, but I would like to solve this without doing that (as you then need to create possibly unnecessary relationships for each node).

2
neo4jcyphergraph-databases

2 Answers

6
votes

For anyone interested, I figured out a way to solve the above using some WITH tricks. This is essentially a solution for creating linked lists in neo4j without having to first create a self referencing relationship.

START root=node(1), item=node(2) 
MATCH root-[old?:LIST_NEXT]->last
CREATE root-[:LIST_NEXT]->item 
WITH item, old, last 
WHERE old IS NOT NULL 
CREATE item-[:LIST_NEXT]->last 
DELETE old

This works by first looking for an existing link relationship, and creating the new one from the root to the item. Then by using WITH we can chain the query to now examine whether or not the matched relationship did in fact exist. If it did, then remove it, and create the remaining link piece from the new item to the old one.