apoc.trigger.toRelationship

Function APOC Full

Signature

apoc.trigger.toRelationship(rel :: RELATIONSHIP, removedRelationshipProperties :: MAP) :: RELATIONSHIP

Input parameters

Name Type Default

rel

RELATIONSHIP

null

removedRelationshipProperties

MAP

null

Usage Examples

This function is intended to be used within an apoc.trigger.install Cypher statement.

If we want to create a 'before' or 'after' trigger query using $deletedRelationships, and retrieve entity information such as the type and/or properties, we cannot use the classic Cypher functions type() and properties(). Instead, we have to leverage virtual relationships through the function apoc.trigger.toRelationship(rel, $removedRelationshipProperties).

For example, to create a new Report node with a list of deleted relationship IDs and the type retrieved for each deleted relationship, we can execute:

CALL apoc.trigger.install(
    'neo4j', 'myTrigger',
    "UNWIND $deletedRelationships as deletedRel
    WITH apoc.trigger.toRelationship(deletedRel, $removedRelationshipProperties) AS deletedRel
    CALL apoc.merge.node(
          ['Report'],
          {type: apoc.rel.type(deletedRel)},
          {created: datetime()},
          {updated: datetime()}
    ) YIELD node AS report
    WITH report, deletedRel
    SET report.deletedIds = coalesce(report.deletedIds, [])+[id(deletedRel)]" ,
    {phase:'before'}
);

Now, let’s create and delete a IN_GENRE relationship between a Movie node and a Genre node:

MERGE (movie:Movie {title: "The White Tiger"})
MERGE (genre:Genre {name: "Triller"})
MERGE (movie)-[IN_GENRE]->(genre);
MATCH (movie:Movie {title: "The White Tiger"})-[r:IN_GENRE]->(genre:Genre {name: "Triller"}) DELETE r;

Finally, let’s check the Report node:

MATCH (report:Report {labels: ['IN_GENRE']})
RETURN report;
Table 1. Results
report

(:Report {"created": "2024-12-12T08:33:27.188000000Z", "deletedIds": [12], "type": "IN_GENRE"})

More documentation of apoc.trigger.toNode