title: Exception because of stale OneToMany relation when merging
body: <p>My entities are like:</p>
  
  <pre><code>class Parent {
     @Id
     long id;
  
     @OneToMany(fetch=FetchType.EAGER)
     @JoinColumn(name="childId", insertable=false, updatable=false)
     Set&lt;Child&gt; children = new HashSet&lt;Child&gt;();
  }
  
  class Child {
     @Id
     long id;
  }
  </code></pre>
  
  <p>I have a Parent entity with some Child loaded in thread A. In thread B, the code deletes Child that belongs to Parent entity in thread A. As a result, thread A encounter javax.persistence.EntityNotFoundException when calling entityManager.merge() because field children contains stale data that has been deleted.</p>
  
  <p>How should I handle this concurrent situation?</p>