title: Unable find Entity .. while trying to save data
body: <p>I'm getting this exception when I try to save a topic with a message: 
  nested exception is javax.persistence.EntityNotFoundException: Unable to find mypackage.model.Message with id fb8d39ea-0094-410d-975a-ea4495781422</p>
  
  <p>Here is the model :</p>
  
  <pre><code>@Entity
  public class Topic {
      @Id
      private String id;
      private String title;
      private String projectName;
      private String author;
      @OneToMany(mappedBy = "topicId")
      private Set&lt;Message&gt; messages;
      public Topic() {
          this.messages = new HashSet&lt;&gt;();
      }
  }
  
  @Entity
  public class Message {
      @Id
      private String id;
      private String author;
      private String content;
      private String topicId;
  }
  </code></pre>
  
  <p>The controller :</p>
  
  <pre><code>@RequestMapping(value = "/projects/{projectSubject}", method = RequestMethod.POST)
      public String createTopic(Model model, @PathVariable("projectSubject") String subject,
                                @RequestParam("title") String title,
                                @RequestParam("message") String messageContent,
                                @RequestParam("author") String author,
                                @RequestParam("projectName") String projectName) {
          Project project = projectService.findBySubject(projectName);
          if(project != null){
              Topic topic = new Topic();
              topic.setId(UUID.randomUUID().toString());
              topic.setAuthor(author);
              topic.setProjectName(projectName);
              topic.setTitle(title);
  
              Message initialPost = new Message();
              initialPost.setId(UUID.randomUUID().toString());
              initialPost.setContent(messageContent);
              initialPost.setAuthor(author);
              topic.getMessages().add(initialPost);
  
              topicService.saveTopic(topic);
         }
         return "topicList";
      }
  </code></pre>
  
  <p>The service :</p>
  
  <pre><code>public void saveTopic(Topic topic) {
          topicRepository.save(topic);
  }
  </code></pre>
  
  <p>The repository :</p>
  
  <pre><code>public interface TopicRepository extends JpaRepository&lt;Topic,String&gt; {}
  </code></pre>