title: Creating unique relationships in Neo4j using py2neo get_or_create
body: <p>I am trying to create a graph using some nodes and relationships using py2noe. say, I'm creating a family tree.</p>
  
  <p>I am creating nodes using <code>get_or_create()</code> so that my script doesn't create duplicates if I supply the same value again.</p>
  
  <p>How can I do the same for a relationship? I can't find any reference to a <code>get_or_create()</code> like function for a relationship.</p>
  
  <p>I want to publish <code>(Joe)-[:son]-&gt;(John)</code>
  the first time it creates 2 nodes joe and john and a link between them.
  if I re run my script, as the nodes are unique, they aren't published but a new relationship is created.</p>
  
  <p>This gives me a graph with 2 nodes and n relationships where n is the number of times I run the script.</p>
  
  <p>I also tried using cypher and I get the same issue. It keeps on creating relationships.</p>
  
  <p>Can anyone suggest me a way to solve this problem?</p>
  
  <p>Thanks.</p>