title: SDN4 - Updating a relationship entity's child causes the relationship to be deleted
body: <p>I am writing some GraphUnit supported integration tests and have noticed odd behavior when I attempt to update the child node an existing relationship points to.</p>
  
  <p>Given an existing relationship with a backing graph that looks like:</p>
  
  <pre><code>(A:ParentNode)-[:SOME_REL {id: 1}]-&gt;(B:ChildNode)
  </code></pre>
  
  <p>If I try to update it by writing code like:</p>
  
  <pre><code>ChildNode newChildNode = new ChildNode();
  existingRelationship.setChild(newChild);
  RelationshipRepository.save(existingRelationship);
  </code></pre>
  
  <p>The resulting graph looks like:</p>
  
  <pre><code>(A:ParentNode)
  (B:ChildNode)
  (C:ChildNode)
  </code></pre>
  
  <p>But I expect it to look like:</p>
  
  <pre><code>(A:ParentNode)-[:SOME_REL {id: 1}]-&gt;(C:ChildNode)
  </code></pre>
  
  <p>The raw log output I see when I execute the GraphRepository <code>save()</code> includes a DELETE statement in the Cypher query that drops the relationship:</p>
  
  <pre><code>10:32:32.218 [main] DEBUG o.n.o.s.r.SessionRequestHandler - {"statements":[{"statement":"MATCH ()-[_0]-&gt;() WHERE id(_0)=0 SET _0+={_0_props}  WITH _0 MATCH ($0)-[_1:`SOME_REL`]-&gt;($1) WHERE id($0)=0 AND id($1)=1 AND id(_1)=0 DELETE _1","parameters":{"_0_props":{"doubleProp":1.0}},"resultDataContents":["row"],"includeStats":false}]}
  10:32:32.219 [main] INFO  o.n.o.session.request.DefaultRequest - POST http://localhost:7475/db/data/transaction/commit, request: {"statements":[{"statement":"MATCH ()-[_0]-&gt;() WHERE id(_0)=0 SET _0+={_0_props}  WITH _0 MATCH ($0)-[_1:`SOME_REL`]-&gt;($1) WHERE id($0)=0 AND id($1)=1 AND id(_1)=0 DELETE _1","parameters":{"_0_props":{"doubleProp":1.0}},"resultDataContents":["row"],"includeStats":false}]}
  </code></pre>
  
  <p>Is this a bug? Or am I not updating an SDN4 relationship entity properly?  Do we have to delete relationships and create them from scratch each time we effectively want to do an update?</p>