title: Can't delete neo4j nodes with a relationship using py2neo
body: <p>I'm learning neo4j through the py2neo module. Modifying the <a href="http://py2neo.org/2.0/intro.html" rel="nofollow">example</a>, I'm confused on why I'm getting an error here. If I want to delete all nodes of the <code>Person</code> type, why can I not iterate through the graph and remove the ones that match my criteria? If the relationship between the nodes is removed, the code runs fine.</p>
  
  <pre><code>from py2neo import Node, Relationship, Graph
  
  g = Graph("http://neo4j:test@localhost:7474/db/data/")
  g.delete_all()
  
  alice = Node("Person", name="Alice")
  bob   = Node("Person", name="Bob")
  g.create(Relationship(alice, "KNOWS", bob)) # Works if this is not present
  
  for x in g.find("Person"):
      print x
      g.delete(x)
  </code></pre>
  
  <p>This fails with the error:</p>
  
  <pre><code>  File "start.py", line 12, in &lt;module&gt;
      g.delete(x)
    ...
  py2neo.error.TransactionFailureException: Transaction was marked as successful, but unable to commit transaction so rolled back.
  </code></pre>