title: Updating a relationship with a merge
body: <p>I'm using the <code>MERGE</code> functionality to create or update a node, and create a relationship between them as follows:</p>
  
  <pre><code>MATCH (p:Part{f:'fff'})
  MERGE (m:Item {p1: {map}.p1})
  ON CREATE SET m = {map}
  ON MATCH SET m += {map}
  MERGE (m)-[:USED_WITH{pri:'High'}]-&gt;(p)
  </code></pre>
  
  <p>However, when I re-run this, but change the property <code>pri</code> on a subsequent run, a new relationship is created. What I want is for the original relationship to be updated.</p>
  
  <p>What's the best practice to do this?</p>