Aggregation functions

This section describes how to write, test and deploy a user-defined aggregation function for Neo4j.

User-defined aggregation functions are functions that aggregate data and return a single result. For a comparison between user-defined procedures, functions and aggregation functions see Introduction.

1. Calling a user-defined aggregation function

User-defined aggregation functions are called in the same way as any other Cypher aggregation function. The function name must be fully qualified, so a function named longestString defined in the package org.neo4j.examples could be called using:

MATCH (p: Person) WHERE p.age = 36  RETURN org.neo4j.examples.longestString(

2. Writing a user-defined aggregation function

User-defined aggregation functions are annotated with @UserAggregationFunction. The annotated function must return an instance of an aggregator class. An aggregator class contains one method annotated with @UserAggregationUpdate and one method annotated with @UserAggregationResult. The method annotated with @UserAggregationUpdate will be called multiple times and enables the class to aggregate data. When the aggregation is done the method annotated with @UserAggregationResult is called once and the result of the aggregation will be returned.

See Values and types for details on values and types.

The correct way to signal an error from within an aggregation function is to throw a RuntimeException.

package example;

import org.neo4j.procedure.Description;
import org.neo4j.procedure.Name;
import org.neo4j.procedure.UserAggregationFunction;
import org.neo4j.procedure.UserAggregationResult;
import org.neo4j.procedure.UserAggregationUpdate;

public class LongestString
    @Description( "org.neo4j.function.example.longestString(string) - aggregates the longest string found" )
    public LongStringAggregator longestString()
        return new LongStringAggregator();

    public static class LongStringAggregator
        private int longest;
        private String longestString;

        public void findLongest(
                @Name( "string" ) String string )
            if ( string != null && string.length() > longest)
                longest = string.length();
                longestString = string;

        public String result()
            return longestString;

2.1. Writing integration tests

Tests for user-defined aggregation functions are created in the same way as those for normal user-defined functions.

Below is a template for testing a user-defined aggregation function that finds the longest string.

package example;

import org.junit.Rule;
import org.junit.Test;
import org.neo4j.driver.v1.*;
import org.neo4j.harness.junit.Neo4jRule;

import static org.hamcrest.core.IsEqual.equalTo;
import static org.junit.Assert.assertThat;

public class LongestStringTest
    // This rule starts a Neo4j instance
    public Neo4jRule neo4j = new Neo4jRule()

            // This is the function to test
            .withAggregationFunction( LongestString.class );

    public void shouldAllowIndexingAndFindingANode() throws Throwable
        // This is in a try-block, to make sure you close the driver after the test
        try( Driver driver = GraphDatabase.driver( neo4j.boltURI() , Config.EncryptionLevel.NONE ).toConfig() ) )
            // Given
            Session session = driver.session();

            // When
            String result = "UNWIND ["abc", "abcd", "ab"] AS string RETURN example.longestString(string) AS result").single().get("result").asString();

            // Then
            assertThat( result, equalTo( "abcd" ) );