title: JPA mappedBy don't persisting entity with relationship
body: <p>For example, I have the following entities:
  Room entity:</p>
  
  <pre><code>@Entity
  public class Room extends Base {
  
  @OneToMany(mappedBy = "room")
  private List&lt;Person&gt; persons;
  //getters and setters
  }
  </code></pre>
  
  <p>And the Person entity:</p>
  
  <pre><code>@Entity
  public class Person extends Base {
  
  @ManyToOne
  private Room room;
  
  private String name;
  //getters and setters
  }
  </code></pre>
  
  <p>If I add person to room's persons list and then persist, the room is added, but person is not. I thought, enough is to add <code>mappedBy</code> parameter to <code>OneToMany</code> annotation, but.. what should I do else?</p>