Neo4j Custom Procedures: How to pass query parameters? - stored-procedures

Question

I am trying to write a custom procedure for the Neo4J GraphDB in accordance to the documentation and the there referenced template. The procedure should ultimately generate a graph projection using the GDSL, for nodes with a certain label that is provided as a procedure parameter. For this it is, of course, necessary to pass the label to the query that is to be executed within the custom procedure. However, I cannot seem to find out how to pass parameters to a query string.
#Procedure(value = "custom.projectGraph")
#Description("Generates a projection of the graph.")
public Stream<ProRecord> projectGraph(#Name("graph") String graph) {
Map<String, Object> params = Map.of (
"graph", graph
);
return tx.execute("call gds.graph.project.cypher(\"$graph\", "
+ "\"MATCH (n:$graph) return id(n) as id\", "
+ "\"MATCH (src:$graph)-[]-(dst:$graph) "
+ "RETURN id(src) AS source, id(dst) AS target\") "
+ "YIELD graphName", params)
.stream()
.map(result -> (String) result.get("graphName"))
.map(ProRecord::new);
}
public static final class ProRecord {
public final String graphName;
public ProRecord(String graphName) {
this.graphName = graphName;
}
}
This code, unfortunately, does not work as intended, throwing the following exception:
Invalid input '$': expected an identifier
I did copy the syntax of prefixing placeholders with $-characters from other examples, as I could not find any hints on the passing of query parameters in the JavaDoc of the library. Is this even the correct documentation for custom neo4j procedures? Am I possibly using the wrong method here to issue my queries? It'd be very kind if someone could lead me into the right direction on that matter.

Answer 1

In general, when you use a string parameter the $param is automatically quoted, unlike the String.format for example.
Therefore there are 2 problems in your query:
\"$graph\" : in this case you are doubly quoting the parameter, try to write only $graph instead
Things like this n:$graph cannot be done unfortunately, the neo4j parameter handling is not able to recognize where to quote and where not, so you could use String.format or concat string with parameters (e.g. "MATCH (n:" + $graph + ") return id(n)...").
So, in short, this piece of code should work in your case:
return tx.execute("call gds.graph.project.cypher($graph, " +
"'MATCH (n:' + $graph + ') return id(n) as id', " +
"'MATCH (src:' + $graph + ')-[]-(dst:' + $graph + ') RETURN id(src) AS source, id(dst) AS target') YIELD graphName",
params)
.stream()
.map(result -> (String) result.get("graphName"))
.map(ProRecord::new);