title: Map from multiple sources DTOs to one target
body: <p>Using MapStruct. Take as example: </p>
  
  <pre><code>class Dto {
      DtoA a;
      DtoB b;
  }
  
  class DtoA {
      Long id;
      //...
  }
  
  class DtoB {
      Long id;
      //...
  }
  
  
  class Entity {
      AB ab;
  }
  </code></pre>
  
  <p>How do I map <code>DtoA</code> and <code>DtoB</code> to AB ?</p>
  
  <p>I tried:</p>
  
  <pre><code>public abstract Entity toEntity(Dto dto);
  
  @Mappings({
              @Mapping(source = "a", target = "ab.a"),
              @Mapping(source = "b", target = "ab.b")
  )}
  public abstract AB toABEntity(DtoA a, DtoB b);
  </code></pre>
  
  <p>Altough code has been generated*, the method <code>toABEntity</code> is not invoked.</p>
  
  <p>*Badly, as it sets first <code>a</code> but then to set <code>b</code> creates a new instance of <code>ab</code>, so <code>a</code> is lost.</p>