How to work with DTO in Spring Data REST projects?

Question

Spring Data REST automates exposing only domain object. But most often we have to deal with Data Transfer Objects. So how to do this in SDR way?

Answer 1

An approach of how to work with DTO in Spring Data REST projects

The working example is here

Entities

Entities must implement the Identifiable interface. For example:

@Entity
public class Category implements Identifiable<Integer> {

    @Id
    @GeneratedValue
    private final Integer id;

    private final String name;

    @OneToMany
    private final Set<Product> products = new HashSet<>();

    // skipped
}

@Entity
public class Product implements Identifiable<Integer> {

    @Id
    @GeneratedValue
    private final Integer id;

    private final String name;

    // skipped
}

Projections

Make a projection interface that repository query methods will return:

public interface CategoryProjection {

    Category getCategory();
    Long getQuantity();
}

It will be a basement for DTO. In this example DTO will represent a Category and the number of Products are belong to it.

Repository methods

Create methods return the projection: a single one, a list of DTO and a paged list of DTO.

@RepositoryRestResource
public interface CategoryRepo extends JpaRepository<Category, Integer> {

    @RestResource(exported = false)
    @Query("select c as category, count(p) as quantity from Category c join c.products p where c.id = ?1 group by c")
    CategoryProjection getDto(Integer categoryId);

    @RestResource(exported = false)
    @Query("select c as category, count(p) as quantity from Category c join c.products p group by c")
    List<CategoryProjection> getDtos();

    @RestResource(exported = false)
    @Query("select c as category, count(p) as quantity from Category c join c.products p group by c")
    Page<CategoryProjection> getDtos(Pageable pageable);
}

DTO

Implement DTO from its interface:

@Relation(value = "category", collectionRelation = "categories")
public class CategoryDto implements CategoryProjection {

    private final Category category;
    private final Long quantity;

    // skipped
}

Annotation Relation is used when Spring Data REST is rendering the object.

Controller

Add custom methods to RepositoryRestController that will serve requests of DTO:

@RepositoryRestController
@RequestMapping("/categories")
public class CategoryController {

    @Autowired private CategoryRepo repo;
    @Autowired private RepositoryEntityLinks links;
    @Autowired private PagedResourcesAssembler<CategoryProjection> assembler;

    /**
    * Single DTO
    */
    @GetMapping("/{id}/dto")
    public ResponseEntity<?> getDto(@PathVariable("id") Integer categoryId) {
        CategoryProjection dto = repo.getDto(categoryId);

        return ResponseEntity.ok(toResource(dto));
    }

    /**
    * List of DTO
    */
    @GetMapping("/dto")
    public ResponseEntity<?> getDtos() {
        List<CategoryProjection> dtos = repo.getDtos();

        Link listSelfLink = links.linkFor(Category.class).slash("/dto").withSelfRel();
        List<?> resources = dtos.stream().map(this::toResource).collect(toList());

        return ResponseEntity.ok(new Resources<>(resources, listSelfLink));
    }

    /**
    * Paged list of DTO
    */
    @GetMapping("/dtoPaged")
    public ResponseEntity<?> getDtosPaged(Pageable pageable) {
        Page<CategoryProjection> dtos = repo.getDtos(pageable);

        Link pageSelfLink = links.linkFor(Category.class).slash("/dtoPaged").withSelfRel();
        PagedResources<?> resources = assembler.toResource(dtos, this::toResource, pageSelfLink);

        return ResponseEntity.ok(resources);
    }

    private ResourceSupport toResource(CategoryProjection projection) {
        CategoryDto dto = new CategoryDto(projection.getCategory(), projection.getQuantity());

        Link categoryLink = links.linkForSingleResource(projection.getCategory()).withRel("category");
        Link selfLink = links.linkForSingleResource(projection.getCategory()).slash("/dto").withSelfRel();

        return new Resource<>(dto, categoryLink, selfLink);
    }
}

When Projections are received from repository we must make the final transformation from a Projection to DTO and 'wrap' it to ResourceSupport object before sending to the client. To do this we use helper method toResource: we create a new DTO, create necessary links for this object, and then create a new Resource with the object and its links.

Result

See the API docs on the Postman site

Singe DTO

GET http://localhost:8080/api/categories/6/dto

{
    "category": {
        "name": "category1"
    },
    "quantity": 3,
    "_links": {
        "category": {
            "href": "http://localhost:8080/api/categories/6"
        },
        "self": {
            "href": "http://localhost:8080/api/categories/6/dto"
        }
    }
}

List of DTO

GET http://localhost:8080/api/categories/dto

{
    "_embedded": {
        "categories": [
            {
                "category": {
                    "name": "category1"
                },
                "quantity": 3,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/6"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/6/dto"
                    }
                }
            },
            {
                "category": {
                    "name": "category2"
                },
                "quantity": 2,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/7"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/7/dto"
                    }
                }
            }
        ]
    },
    "_links": {
        "self": {
            "href": "http://localhost:8080/api/categories/dto"
        }
    }
}

Paged list of DTO

GET http://localhost:8080/api/categories/dtoPaged

{
    "_embedded": {
        "categories": [
            {
                "category": {
                    "name": "category1"
                },
                "quantity": 3,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/6"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/6/dto"
                    }
                }
            },
            {
                "category": {
                    "name": "category2"
                },
                "quantity": 2,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/7"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/7/dto"
                    }
                }
            }
        ]
    },
    "_links": {
        "self": {
            "href": "http://localhost:8080/api/categories/dtoPaged"
        }
    },
    "page": {
        "size": 20,
        "totalElements": 2,
        "totalPages": 1,
        "number": 0
    }
}