title: In Cypher, how can I create a relationship if it doesn't exist; update property if it does
body: <p>In Cypher in Neo4J, given two nodes, if there's no relationship between them, I'd like to create a relationship (of type Foo) with a weight property of one.  If this relationship already exists, I'd like to increment its weight property.</p>
  
  <p>Is there a good way to do this in a single Cypher query?  Thanks!</p>
  
  <p>Edit: Some additional details:
  The nodes are already created, unique, and in an index.</p>