title: "ids for this class must be manually assigned before calling save()" with string id
body: <p>I got exception when I'm writing hibernate in memory test.</p>
  
  <blockquote>
    <p>org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save():</p>
  </blockquote>
  
  <pre><code>@Entity
  public Account{
       @Id
       String num;
  }
  </code></pre>
  
  <p>First, this id of pojo is String and I cannot change it, I don't have access to DB and change pojo, all I can do is create in-memory tests.</p>
  
  <p>Then, before I save this pojo, I filled every field in this pojo, still it throws this exception. And because of in-memory test, the in memory DB is empty, there're not conflict id there.</p>
  
  <p>Any idea, what else can cause this exception?</p>