Modeling arrows/relationships as nodes in Neo4j

Question

Relationship/Arrows in Neo4j can not get more than one type/label (see here, and here). I have a data model that edges need to get labels and (probably) properties. If I decide to use Neo4j (instead of OriendDB which supports labeled arrow), I think I would have then two options to model an arrow, say f, between two nodes A and B:

1) encode an arrow f as a span, say A<--f-->B, such that f is also a node and --> and <-- are arrows.

or

2) encode an arrow f as A --> f -->B, such that f is a node again and two --> are arrows.

Though this seems to be adding unnecessary complexity on my data model, it does not seem to be any other option at the moment if I want to use Neo4j. Then, I am trying to see which of the above encoding might fit better in my queries (queries are the core of my system). For doing so, I need to resort to examples. So I have two question:

First Question:

part1) I have nodes labeled as Person and father, and there are arrows between them like Person<-[:sr]-father-[:tr]->Person in order to model who is father of who (tr is father of sr). For a given person p1 how can I get all of his ancestors.

part2) If I had Person-[:sr]->father-[:tr]->Person structure instead, for modeling father relationship, how the above same query would look like.

This is answered here when father is considered as a simple relationship (instead of being encoded as a node)

Second Question:

part1) I have nodes labeled as A nodes with the property p1 for each. I want to query A nodes, get those elements that p1<5, then create the following structure: for each a1 in the query result I create qa1<-[:sr]-isA-[:tr]->a1 such that isA and qa1 are nodes.

part2) What if I wanted to create qa1-[:sr]->isA-[:tr]->qa1 instead?

This question is answered here when isA is considered as a simple arrow (instead of being modeled as a node).

FrobberOfBits FrobberOfBits · Accepted Answer · 2015-05-01T16:12:12

First, some terminology; relationships don't have labels, they only have types. And yes, one type per relationship.

Second, relative to modeling, I think the direction of the relationship isn't always super important, since with neo4j you can traverse it both ways easily. So the difference between A-->f-->B and A<--f-->B I think should be entirely driven what what makes sense semantically for your domain, nothing else. So your options (1) and (2) at the top seem the same to me in terms of overall complexity, which brings me to point #3:

Your main choice is between making a complex relationship into a node (which I think we're calling f here) or keeping it as a relationship. Making "a relationship into a node" is called reification and I think it's considered a fairly standard practice to accommodate a number of modeling issues. It does add complexity (over a simple relationship) but adds flexibility. That's a pretty standard engineering tradeoff everywhere.

So with all of that said, for your first question I wouldn't recommend an intermediate node at all. :father is a very simple relationship, and I don't see why you'd ever need more than one label on it. So for question one, I would pick "neither of the options you list" and would instead model it as (personA)-[:father]->(personB). More simple. You'd query that by saying

   MATCH (personA { name: "Bob"})-[:father]->(bobsDad) RETURN bobsDad

Yes, you could model this as (personA)-[:sr]->(fatherhood)-[:tr]->(personB) but I don't see how this gains you much. As for the relationship direction, again it doesn't matter for performance or query, only for semantics of whatever :tr and :sr are supposed to mean.

I have nodes labeled as A nodes with the property p1 for each. I want to query A nodes, get those elements that p1<5, then create the following structure: for each a1 in the query result I create qa1<-[:sr]-isA-[:tr]->a1 such that isA and qa1 are nodes.

That's this:

MATCH (aNode:A)
WHERE aNode.p1 < 5
WITH aNode
MATCH (qa1 { label: "some qa1 node" })
CREATE (qa1)<-[:sr]-(isA)-[:tr]->aNode;

Note that you'll need to adjust the criteria for qa1 and also specify something meaningful for isA.

What if I wanted to create qa1-[:sr]->isA-[:tr]->qa1 instead?

It should be trivial to modify that query above, just change the direction of the arrows, same query.

Modeling arrows/relationships as nodes in Neo4j

1 Answers