Can a graph database edge have multiple starting nodes?

Question

I am designing a graph database for eligibility rules. Some eligibility rules require that a user select 2 particular products (Product A and Product B) to qualify for Product C.

Is it possible to create a graph edge with 2 starting nodes?

I would think this would break what I think is the fundamental building block of a graph db - its adjacency list. But if this was possible, it would be very powerful for my application.

Update 6/16

More specifically, I'm looking to create a directed edge with 2 starting nodes, and 1 ending node. So, in biz rules terms: IF Node=A AND Node=B THEN Node=C. The real world relationship is this: If customer buys Product A and Product B, then customer qualifies for Product C.

Answer 1

Yes you can have multiple starting nodes in Neo4j, not sure about other graph db.

START a=node(0), b=node(1)
RETURN a,b

You should refer to http://docs.neo4j.org/chunked/stable/query-start.html for more details. Starting from Neo4j 2.0 start node is optional, Cypher will try and infer starting points from your query based on label and where clause.

Edit

I have edited the answer based on the updated question. What you need is a hypergraph. As Wes Freeman mentioned, to model a hypergraph Neo4j you will need to create an intermediate node that connects your other two nodes and the the third node. In you scenario a user will have a PURCHASED relationship with the two products(A and B) kinda like (:User {Id: 1})-[:PURCHASED]->(:Product {Name:A}). Then you will have to create an intermediate node like ProductQualifier (I am very bad at naming things) having a relationship from user like (:User {Id:1})-[:QUALIFIES]->(:ProductQualifier {Id:1}). The Product qualifier will then have 3 relations, two to Product A and B respectively and a third to Product C,

(:Product {Name: 'B'})<-[:HAS]-(:ProductQualifier {Id:1})-[:HAS]->(:Product {Name: 'A'})

(ProductQualifier {Id:1}-[:ELIGIBLE]->(:Product {Name: 'C'})

This ought to do what you want.

A second approach that you can take is use a database that inherently supports hypergraphs, something like Hypergraphdb, thus discarding the burden of creating extra node. I haven't had any occasion to use it though I wanted to try it out for quite some time, so I don't know in much details about its API's or its limitations, however it is fairly well known graph database.

Note: As mentioned I am very bad at naming things. You should probably change the label names to more suitable to your business model.

Answer 2

Usually, to model a hypergraph in Neo4j, you end up creating an intermediate "group node" that connects all of the nodes you want to connect, then bridging off of that node to the other node. It's not a true hypergraph, but rather a representation of a hypergraph using the tools provided.

Here's an example: http://www.markhneedham.com/blog/2013/10/22/neo4j-modelling-hyper-edges-in-a-property-graph/