ModelMapper Set attribute of nested object

Question

Is there a way configure model mapper to automatically map parent id to parent ids in nested child object?

Parent entity

@Entity
@Table(name = "parent")
public class ParentEntity {

    @Id
    @Column(name = "id")
    private Long id;

    @Basic
    @Column(name = "name")
    private String name;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "parent_id", referencedColumnName = "id")
    private List<ChildEntity> child;

Child entity

@Entity
@Table(name = "child")
public class ChildEntity {

    @Id
    @Column(name = "id")
    private Long id;

    @Basic
    @Column(name = "parent_id")
    private Long parentId;

    @Basic
    @Column(name = "name")
    private String name;

Parent DTO

public class ParentDto {
    
    private Long id;
    private String name;
    private List<ChildDto> children;

Child DTO

public class ChildDto {

    private Long id;
    private Long parentId;
    private String name;

Currently Im using model mapper and it conversion works for the most part, and it creates a ParentEntity object with a list of ChildEntity object. Now Im looking for a way to populate the "parentId" field in each ChildEntity with modelmapper. Thanks in advance!

/*
{
  id: 1,
  name: "dad",
  children: [
    {
      id:10,
      name: "child",
    },
    {
      id:20,
      name: "child2",
    }
  ]
}
*/
modelMapper.map(parentDto, ParentEntity.class)

Answer 1

The challenge is that when ChildDto is mapped you must have access to the ParentDto that has the List children in which the ChildDto to be mapped is added. Otherwise the mapper does not know about that id.

Luckily you can access ParentDto with a little help from a org.modelmapper.Converter. Implement this interface like:

public Converter<ChildDto, ChildEntity> converter = new Converter<>() {
    private final ModelMapper mm = new ModelMapper();
    @Override
    public ChildEntity convert(MappingContext<ChildDto, ChildEntity> context) {
        // map it first normally
        ChildEntity ce = mm.map(context.getSource(), ChildEntity.class);
        try {
            // 1st parent is the List 2nfd is the ParentDto
            ParentDto parentDto = (ParentDto)context.getParent().getParent().getSource();
            ce.setParentId(parentDto.getId());
        } catch (Exception e) {
            // well, maybe it was not ParentDto that expected to be the grandparent.
        }
        return ce;
    }
};

Then use it like:

ModelMapper mm = new ModelMapper();
mm.addConverter(converter);

and ChildDto that has no parentId should still be mapped to ChildEntity with parentId.