java neo4j check if a relationship exist

Question

excluse if this is a duplicate, although I did not find the answer so far.

I have an application that creates nodes and relationships via cypher statement against the REST-API. I create relationships with the below code:

public URI createRelationship(GraphNodeTypes sourceType, URI sourceNode, 
                        GraphNodeTypes targetType, URI targetNode,
    GraphRelationshipTypes relationshipType, String[] jsonAttributes) {
URI relationShipLocation = null;

String cypherArt = getNodeIdFromLocation(sourceNode)+"-[:"+relationshipType+"]->"+getNodeIdFromLocation(targetNode);

logger.info("creating relationship ({}:{}) -[:{}]-> ({}:{})", 
                                sourceType,
                                getNodeIdFromLocation(sourceNode), 
                                relationshipType,
                                targetType,
                                getNodeIdFromLocation(targetNode));

try {
    URI finalUrl = new URI( sourceNode.toString() + "/relationships" );
    String cypherStatement = generateJsonRelationship( targetNode,
                                                        relationshipType, 
                                                        jsonAttributes );

    logger.trace("sending CREATE RELATIONSHIP cypher as {} to endpoint {}", cypherStatement, finalUrl);
    WebResource resource = Client.create().resource( finalUrl );

    ClientResponse response = resource
            .accept( MediaType.APPLICATION_JSON )
            .type( MediaType.APPLICATION_JSON )
            .entity( cypherStatement )
            .post( ClientResponse.class );

    String responseEntity = response.getEntity(String.class).toString();
    int responseStatus = response.getStatus();

    logger.trace("POST to {} returned status code {}, returned data: {}",
            finalUrl, responseStatus,
            responseEntity);

    // first check if the http code was ok
    HttpStatusCodes httpStatusCodes = HttpStatusCodes.getHttpStatusCode(responseStatus);
    if (!httpStatusCodes.isOk()){
        if (httpStatusCodes == HttpStatusCodes.FORBIDDEN){
            logger.error(HttpErrorMessages.getHttpErrorText(httpStatusCodes.getErrorCode()));
        } else {
            logger.error("Error {} sending data to {}: {} ", response.getStatus(), finalUrl, HttpErrorMessages.getHttpErrorText(httpStatusCodes.getErrorCode()));
        }
    } else {
        JSONParser reponseParser = new JSONParser();
        Object responseObj = reponseParser.parse(responseEntity);
        JSONObject jsonResponseObj = responseObj instanceof JSONObject ?(JSONObject) responseObj : null;
        if(jsonResponseObj == null)
            throw new ParseException(0, "returned json object is null");

        //logger.trace("returned response object is {}", jsonResponseObj.toString());
        try {
            relationShipLocation = new URI((String)((JSONObject)((JSONArray)((JSONObject)((JSONArray)((JSONObject)((JSONArray)jsonResponseObj.get("results")).get(0)).get("data")).get(0)).get("rest")).get(0)).get("self"));
        } catch (Exception e) {
            logger.warn("CREATE RELATIONSHIP statement did not return a self object, returning null -- error was {}", e.getMessage());
            relationShipLocation = null;
        }
    }
} catch (Exception e) {
    logger.error("could not create relationship ");
}
return relationShipLocation;
}

private static String generateJsonRelationship( URI endNode,
    GraphRelationshipTypes relationshipType, String[] jsonAttributes ) {
StringBuilder sb = new StringBuilder();
sb.append( "{ \"to\" : \"" );
sb.append( endNode.toString() );
sb.append( "\", " );

sb.append( "\"type\" : \"" );
sb.append( relationshipType.toString() );
if ( jsonAttributes == null || jsonAttributes.length < 1 ){
    sb.append( "\"" );
} else {
    sb.append( "\", \"data\" : " );
    for ( int i = 0; i < jsonAttributes.length; i++ ) {
        sb.append( jsonAttributes[i] );
        if ( i < jsonAttributes.length - 1 ){
            // Miss off the final comma
            sb.append( ", " );
        }
    }
}

sb.append( " }" );
return sb.toString();
}

My problem is that I would like to check if a given relationship of given type already exists between two nodes PRIOR creating it.

Can someone tell me, how to query for a relationship???

With nodes I do a MATCH like this:

 MATCH  cypher {"statements": [ {"statement": "MATCH (p:SOCIALNETWORK {sn_id: 'TW'} ) RETURN p", "resultDataContents":["REST"]} ] } 

against the endpoint

 http://localhost:7474/db/data/transaction/<NUMBER>

How would I construct the statement to check for a relationship, say between node 6 and 5 or whatever?

Thanks in advance,

Chris

Answer 1

You might want to consider doing this through cypher, and using the MERGE/ON CREATE/ON MATCH keywords.

For example, you could do something like this:

create (a:Person {name: "Bob"})-[:knows]->(b:Person {name: "Susan"});

MATCH  (a:Person {name: "Bob"}), (b:Person {name: "Susan"}) 
MERGE (a)-[r:knows]->(b) 
ON CREATE SET r.alreadyExisted=false 
ON MATCH SET r.alreadyExisted=true 
RETURN r.alreadyExisted;

This MATCH/MERGE query that I provide here will return true or false, depending on whether the relationship already existed or not.

Also, FWIW it looks like the code you're using to accumulate JSON via StringBuilder objects is likely to be ponderous and error prone. There are plenty of good libraries like Google GSON that will do JSON for you, so you can create JSON objects, arrays, primitives, and so on -- then let the library worry about serializing it properly to a string . This tends to make your code a lot cleaner, easier to maintain, and when you mess something up about your JSON formatting (we all do), it's way easier to find than when you accumulate strings like that.

Answer 2

In Java

Relationship getRelationshipBetween(Node n1, Node n2) { // RelationshipType type, Direction direction
    for (Relationship rel : n1.getRelationships()) { // n1.getRelationships(type,direction)
       if (rel.getOtherNode(n1).equals(n2)) return rel;
    }
    return null;
}