CODEWITHSUNDEEP
mergeneo4jcypher
sandrooco
sandrooco

Reputation: 8776

Cypher how to MERGE without re-creating existing nodes

I have to update some properties on en entity (we extracted some user properties to a separate entity). Now I'm trying to write a query that updates the existing data. This is what I have so far:

// Match the user and it's settings
MATCH (s:UserSettings)-[:SETTINGS_FROM]->(u:User)

// Set the properties of the settings node to their old value from user entity
SET s.`settings.key1` = u.key1
SET s.`settings.key2` = u.key2
SET s.`settings.key3` = u.key3

// Remove the keys on the user entity
SET u.key1 = null
SET u.key2 = null
SET u.key3 = null

RETURN s, u;

This works for users that already have a UserSettings node attached, but not for users without. I tried to replace MATCH with MERGE - this will create a new user although all users are available in the db. What am I doing wrong?

Upvotes: 0

Views: 684

Answers (1)

fbiville
fbiville

Reputation: 8970

Using MERGE on a path means that if any of the path elements is missing, the whole pattern will be created. Usually, you want to MERGE specific nodes and relationships, not a whole path at once.

Assuming:

  1. the user nodes are always present
  2. the settings nodes are always created at the same time as their SETTINGS_FROM relationship (otherwise, you'd have to MERGE first on the settings node pattern AND then on the relationship)

You should run something like:

MATCH (u:User)
MERGE (s:UserSettings)-[:SETTINGS_FROM]->(u)
ON CREATE
   SET s.`settings.key1` = u.key1
   SET s.`settings.key2` = u.key2
   SET s.`settings.key3` = u.key3
REMOVE u.key1 // those removes are not part of ON CREATE (see https://github.com/neo4j/neo4j/issues/6172)
REMOVE u.key2
REMOVE u.key3

Upvotes: 1

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