Return non-matching nodes while creating relationships Neo4j Cypher

Question

I wrote a script to to batch create a bunch of relationship in neo4j. Here is the cypher:

:param batch => [{startId: 'abc123', endId: 'abc321'}, {startId: 'abc456', endId: 'abc654']
UNWIND $batch as row
MATCH (from {id: row.startId}
MATCH (to {id: row.endId}
CREATE (from)-[rel:HAS]->(to)
RETURN rel

The problem that there might be some startId/endId entries that don't match any nodes and are silently ignore. Is there a way to return the list of rows that don't match any nodes and create the relationship for the nodes that do match?

I tried OPTIONAL MATCH to fail-fast as soon an id doesn't find a startId/endId however, the query execution was really slow.

cybersam cybersam · Accepted Answer · 2020-07-22T19:39:21

First of all, you should always try to specify a label for the node that is used to kick off a MATCH (unless the MATCH pattern uses any already-bound nodes). Otherwise, every single node in the DB must be scanned. In addition, you should consider using indexes to speed up your MATCHs (but, again, you'd need to specify the labels).

Here is a query that uses the APOC procedure apoc.do.when to create a new relationship when appropriate. It returns each row and the corresponding new relationship (or NULL if either node is not found):

UNWIND $batch as row
OPTIONAL MATCH (from:Foo {id: row.startId})
OPTIONAL MATCH (to:Foo {id: row.endId})
CALL apoc.do.when(
  from IS NOT NULL AND to IS NOT NULL,
  'CREATE (from)-[rel:HAS]->(to) RETURN rel',
  'RETURN NULL AS rel',
  {from: from, to: to}) YIELD value
RETURN row, value.rel AS rel

Return non-matching nodes while creating relationships Neo4j Cypher

1 Answers