How to delete relationships with properties by passing parameters?

Question

env:

1. Neo4j-4.4.x
2. neo4j-jdbc-driver 4.0.9

Image that I execute the following cypher to get a ‘teacher’ node and a ‘student’ node, the relationship between them is ‘educate’, and the relationship has a year attribute.

cypher:

merge (subject:Teacher{teacher_id: 'teacher_001'})  
merge (object:Student{student_id:'student_001'})  
merge (subject)-[relation:educate]->(object) set relation.year='3'  return subject,relation,object

figure-1:

Why does my following approach with the help of parameters maps not work?

@Test
public void testDeleteEdgeWithProperties_001() {
    Driver driver = GraphDatabase.driver("neo4j://localhost:7687", AuthTokens.basic("neo4j", "neo4j123"));
    Session session = driver.session();
    Transaction transaction = session.beginTransaction();

    String relationModelType = "educate";
    String subjectModelType = "Teacher";
    String subjectKey = "teacher_id";
    String subjectId = "teacher_001";
    String objectModelType = "Student";
    String objectKey ="student_id";
    String objectId = "student_001";

    Map<String, Object> properties = new HashMap<>();
    properties.put("year","3");
    Map<String, Object> params = new HashMap<>();
    params.put("relation_prop", properties == null ? new HashMap<>() : properties);

    // 删除
    String conditionDeleteCypher = " WHERE relation = $relation_prop";
    String cypher =  " MATCH (subject:" + subjectModelType + "{" + subjectKey + ": '" + subjectId + "'}) " +
            " MATCH (object:" + objectModelType + "{" + objectKey + ":'" + objectId + "'}) " +
            " MATCH (subject)-[relation:" + relationModelType +"]->(object) " +
            conditionDeleteCypher +
            " DELETE relation";

    System.out.println(cypher);
    transaction.run(cypher, params);
    transaction.commit();
    transaction.close();
}

The cypher printed by the above method is as follows：

MATCH (subject:Teacher{teacher_id: 'teacher_001'})  MATCH (object:Student{student_id:'student_001'})  MATCH (subject)-[relation:educate]->(object)  WHERE relation = $relation_prop DELETE relation

However, the above cypher can be executed successfully in the neo4j browser (see figure-2 below). What's wrong with me? Appreciate any helpful responses

figure-2:

I tried several ways to set the parameter, but none of them worked.

String conditionDeleteCypher = " WHERE relation = $relation_prop";

String conditionDeleteCypher = " WHERE relation =+ $relation_prop";

String conditionDeleteCypher = " WHERE relation += $relation_prop";

The @Test code I provided is completely executable, no error message. After execution, the neo4j browser can still find the target relationship, indicating that the method was executed but did not take effect.

Answer 1

The query generated by your code, is logically incorrect:

MATCH (subject:Teacher{teacher_id: 'teacher_001'})  
MATCH (object:Student{student_id:'student_001'})  
MATCH (subject)-[relation:educate]->(object)  WHERE relation = $relation_prop 
DELETE relation

In this query, you are comparing the whole Neo4j Relationship object, to a query object, which is like this:

{
  relation_prop: {
     year: "3"
  }
}

This won't work because of two reasons:

1. relation_prop is not a valid relation property.
2. There are a lot of other internal properties, part of a relationship.

In the query you ran on Neo4j Browser, you are only checking the value of the year property. To fix it you can try this:

@Test
public void testDeleteEdgeWithProperties_001() {
    Driver driver = GraphDatabase.driver("neo4j://localhost:7687", AuthTokens.basic("neo4j", "neo4j123"));
    Session session = driver.session();
    Transaction transaction = session.beginTransaction();

    String relationModelType = "educate";
    String subjectModelType = "Teacher";
    String subjectKey = "teacher_id";
    String subjectId = "teacher_001";
    String objectModelType = "Student";
    String objectKey ="student_id";
    String objectId = "student_001";

    Map<String, Object> properties = new HashMap<>();
    properties.put("year","3");

    // 删除
    String conditionDeleteCypher = " WHERE relation.year = $year";
    String cypher =  " MATCH (subject:" + subjectModelType + "{" + subjectKey + ": '" + subjectId + "'}) " +
            " MATCH (object:" + objectModelType + "{" + objectKey + ":'" + objectId + "'}) " +
            " MATCH (subject)-[relation:" + relationModelType +"]->(object) " +
            conditionDeleteCypher +
            " DELETE relation";

    System.out.println(cypher);
    transaction.run(cypher, properties);
    transaction.commit();
    transaction.close();
}

Answer 2

You can check the official Neo4J java documentation with many examples. Especially these one :

Map<String,Object> params = new HashMap<>();
params.put( "name", "Johan" );

String query =
"MATCH (n:Person)" + "\n" +
"WHERE n.name = $name" + "\n" +
"RETURN n";

Result result = transaction.execute( query, params );

And make your minimum use case lighter removing any useless code.