Java to JSON serialization with Jackson PTH and Spring Data MongoDB DBRef generates extra target property

Question

When serializing from Java to JSON, Jackson generates an extra target property for referenced entities when using the Spring Data MongoDB @DBRef annotation with lazy loading and Jackson's polymorphic type handling. Why does this occur, and is it possible to omit the extra target property?

Code Example

@Document(collection = "cdBox")
public class CDBox {
  @Id
  public String id;

  @DBRef(lazy = true)
  public List<Product> products;
}

@Document(collection = "album")
public class Album extends Product {
  @DBRef(lazy = true)
  public List<Song> songs;
}

@Document(collection = "single")
public class Single extends Product {
  @DBRef(lazy = true)
  public List<Song> songs;
}

@Document(collection = "song")
public class Song {
  @Id
  public String id;

  public String title;
}

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME,
                    property = "productType",
                    include = JsonTypeInfo.As.EXTERNAL_PROPERTY)
@JsonSubTypes(value = {
    @JsonSubTypes.Type(value = Single.class),
    @JsonSubTypes.Type(value = Album.class)
})
public abstract class Product {
  @Id
  public String id;
}

Generated JSON

{
  "id": "someId1",
  "products": [
    {
      "id": "someId2",
      "songs": [
        {
        "id": "someId3",
        "title": "Some title",
        "target": {
          "id": "someId3",
          "title": "Some title"
          }
        }
      ]
    }
  ]
}

Answer 1

The Target field is added by Spring Data because it is a lazy collection. So it is like datahandler etc. in Hibernate for JPA.

Option1: To ignore them you just have to add @JsonIgnoreProperties(value = { "target" }) on class level

@Document(collection = "song")
@JsonIgnoreProperties(value = { "target" })
public class Song {
 ...
}

Option2: Make the Collection not lazy