crud mongodb returns null

I am doing a crud with SpringBoot and mongoDb in my local, I have added a record from the mongo console ->

db.articulo.save({"_id":ObjectId("90e"),"idPromocion":1,"numArticulos":"3","articulos":[{"nombre":"silla","color":"rojo","precio":"10€"},{"nombre":"mesa","color":"azul","precio":"2€"}],"entrega":"sabado"}

And now to make the crud I have created the model

@Document
public class Articulo {

    private Integer idPromocion; 
    private Articulos art; 

    public Articulo(){}

    public Articulo(Integer idPromocion,Articulos art){
        this.idPromocion=idPromocion;
        this.art=art;
    }
    getters.setter...
 }


 public class Articulos {
  private String nombre; 
  private String color; 
  private String precio; 

  constructor, getter, setters
 }
 ---------------

the repository

    @Repository
    public interface ArticuloRepository extends CrudRepository<Articulo, String> 
 { }

Implementation

@Autowired
private ArticuloRepository repositorio;

     List<Articulo> list= (List<Articulo>) repositorio.findAll();

Later in the execution I have checked the data of the list and I find the data to null, apparently it does not recover them correctly. I have another collection with a simpler structure and it does retrieve them for me.