How to find all nodes connected to certain nodes in a directed graph in Neo4j without infinite loops?

Question

I'm using Neo4j. I want to find all nodes that connect (by one direction) to certain nodes targets in a one-way directed graph (maybe with loops). For example:

targets: [4]
1->2->3->[4]->2->...
1->5->6->7->5->...

Now 4 is the target node, but there can be more than one target node. I want to find 1,2,3,4 as the result, since they can connect with target node t , while 5,6,7 cannot.

I would like to know how to make it by Cypher in Neo4j.

My first thought was:

MATCH (target) WHERE id(target) IN [4]
MATCH p=(a)-[:Rel*]->(target)
MATCH (a)-[r:Rel]->(c) WHERE a IN nodes(p) AND c IN nodes(p)
RETURN a,c

It worked well when there was no loop, but when there're loops the [:Rel*] will run infinitely, for example finding paths to 4 in 5,6,7 .

I had an idea (inspired by Label Propagation) to solve it:

1. Mark target nodes 'red'.
2. Mark all nodes which has a outgoing relationship to 'red' nodes 'red', too.
3. Repeat Step 2 until there're no more new 'red' nodes.
4. All 'red' nodes (including target nodes) are in the same result subgraph now.

But I cannot figure out how to write it in Cypher, as it contains recursions, and I'm having difficulties writing those Java User-defined procedures for Neo4j.

Can it be written in Cypher, or are there already any solutions for tasks like this one? I didn't find useful tools in Apoc or Graph Data Science libraries.

Any help would be appreciated, thanks!

Answer 1

One way is using apoc.path.expandConfig . This is part of the apoc plugin that you can easily enable on your neo4j db. Using it, you can run something like:

MATCH (target) WHERE id(target) IN [4]
CALL apoc.path.expandConfig(target, {
    relationshipFilter: "<Rel",
    maxLevel: 10 //This is optional
})
YIELD path
RETURN path

On the first line you find your target nodes, as you did. The relationshipFilter "<REL" will allow traversing only on nodes pointing at the target direction.