Cypher Neo4J - CASE Expression with MERGE

Question

I'm trying to implement the logic in Cypher where, based on a particular condition (CASE Statement), I would create some nodes and relationships; the code is as below

MATCH (g:Game)-[:PLAYER]->(u:User)-[r1:AT]->(b1:Block)-[:NEXT]->(b2:Block) 
WHERE g.game_id='G222' and u.email_id = '[email protected]' and b1.block_id='16' 
SET r1.status='Skipped', r1.enddate=20141225
WITH u, b2,b1, g, r1
SET b1.test = CASE b2.fork 
WHEN 'y' THEN
     MERGE (u)-[r2:STAGE {startdate:20141225, enddate:'99999999', status:'InProgress'}]->(b2     {fork:'fail'}) RETURN 1
ELSE 
     MERGE (u)-[r2:STAGE {startdate:20141225, enddate:'99999999', status:'InProgress'}]->(b2)   RETURN 2
END
WITH u, g
MATCH (u)-[:TIME]->(h:Time)<-[:TIME]-(g)
SET h.after = 0
SET h.before = h.before + 1

In this query there is a merge statement within the WHEN 'y' THEN, this query throws an error:

Invalid input ']': expected whitespace or a relationship pattern (line 7, column 82) "MERGE (u)-[r2:STAGE {startdate:20141225, enddate:'99999999', status:'InProgress'}]->(b2 {fork:'fail'}) RETURN 1"

Basically I'm trying to create a relationship based on a property i.e. a MERGE within a CASE statement, I tried different ways to get this working like doing a return so that case when returns some value etc. but nothing worked so far.

What could be the issue with this query?

Answer 1

To do conditional write operations you need to use the FOREACH trick. Using CASE you either return a one element array or a empty one. FOREACH iterates over the CASE expression and therefore conditionally executes the action. If you want an ELSE part as well you need to have a another FOREACH using the inverse condition in the CASE. As an example, instead of

WHEN 'y' THEN
   MERGE (u)-[r2:STAGE {startdate:20141225, enddate:'99999999', status:'InProgress'}]->(b2     {fork:'fail'}) RETURN 1
ELSE 
    MERGE (u)-[r2:STAGE {startdate:20141225, enddate:'99999999', status:'InProgress'}]->(b2)   RETURN 2
END

use

FOREACH(ignoreMe IN CASE WHEN 'y' THEN [1] ELSE [] END | 
    MERGE (u)-[r2:STAGE {startdate:20141225, enddate:'99999999', status:'InProgress'}]->(b2 {fork:'fail'})
)
FOREACH(ignoreMe IN CASE WHEN NOT 'y' THEN [1] ELSE [] END | 
    MERGE (u)-[r2:STAGE {startdate:20141225, enddate:'99999999', status:'InProgress'}]->(b2)
)

See also Mark's blog post on this.