Spring Data JPA and Querydsl to fetch subset of columns using bean/constructor projection

I have an entity class as below:

@Entity
public class UserDemo implements Serializable {

    @Id
    private Long id;

    private String username;

    private String createdBy;
    @Version
    private int version;

    /***
     *
     * Getters and setters
     */
}

Using Spring Data JPA and Querydsl how do I fetch a page of UserDemo with only id and username properties populated? I need to use paging as well as searching. In short I would like to achieve the same result as

Page<UserDemo> findAll(Predicate predicate, Pageable pageable);

but with limited field of UserDemo populated.

Looks like custom repository implementation is the way to go for now until something similar available in spring data.

I have gone through http://static.springsource.org/spring-data/data-jpa/docs/current/reference/html/repositories.html#repositories.custom-implementations

Here is my implementation which works. However it would be good to have this method available directly in Spring-Data-JPA

Step 1: Intermediate interface for shared behavior

public interface CustomQueryDslJpaRepository <T, ID extends Serializable>
        extends JpaRepository<T, ID>, QueryDslPredicateExecutor<T> {
    /**
     * Returns a {@link org.springframework.data.domain.Page} of entities matching the given {@link com.mysema.query.types.Predicate}.
     * This also uses provided projections ( can be JavaBean or constructor or anything supported by QueryDSL
     * @param constructorExpression this constructor expression will be used for transforming query results
     * @param predicate
     * @param pageable
     * @return
     */
    Page<T> findAll(FactoryExpression<T> factoryExpression, Predicate predicate, Pageable pageable);
}

Step 2: Implementation of intermediate interface

public class CustomQueryDslJpaRepositoryImpl<T, ID extends Serializable> extends QueryDslJpaRepository<T, ID>
        implements CustomQueryDslJpaRepository<T, ID> {

    //All instance variables are available in super, but they are private
    private static final EntityPathResolver DEFAULT_ENTITY_PATH_RESOLVER = SimpleEntityPathResolver.INSTANCE;

    private final EntityPath<T> path;
    private final PathBuilder<T> builder;
    private final Querydsl querydsl;

    public CustomQueryDslJpaRepositoryImpl(JpaEntityInformation<T, ID> entityInformation, EntityManager entityManager) {
        this(entityInformation, entityManager, DEFAULT_ENTITY_PATH_RESOLVER);
    }

    public CustomQueryDslJpaRepositoryImpl(JpaEntityInformation<T, ID> entityInformation, EntityManager entityManager,
                                 EntityPathResolver resolver) {

        super(entityInformation, entityManager);
        this.path = resolver.createPath(entityInformation.getJavaType());
        this.builder = new PathBuilder<T>(path.getType(), path.getMetadata());
        this.querydsl = new Querydsl(entityManager, builder);
    }

    @Override
    public Page<T> findAll(FactoryExpression<T> factoryExpression, Predicate predicate, Pageable pageable) {
        JPQLQuery countQuery = createQuery(predicate);
        JPQLQuery query = querydsl.applyPagination(pageable, createQuery(predicate));

        Long total = countQuery.count();
        List<T> content = total > pageable.getOffset() ? query.list(factoryExpression) : Collections.<T> emptyList();

        return new PageImpl<T>(content, pageable, total);
    }
}

Step 3: Create a custom repository factory to replace the default

public class CustomQueryDslJpaRepositoryFactoryBean<R extends JpaRepository<T, I>, T, I extends Serializable>
        extends JpaRepositoryFactoryBean<R, T, I> {

    protected RepositoryFactorySupport createRepositoryFactory(EntityManager entityManager) {

        return new CustomQueryDslJpaRepositoryFactory(entityManager);
    }
    private static class CustomQueryDslJpaRepositoryFactory<T, I extends Serializable> extends JpaRepositoryFactory {

        private EntityManager entityManager;

        public CustomQueryDslJpaRepositoryFactory(EntityManager entityManager) {
            super(entityManager);
            this.entityManager = entityManager;
        }

        protected Object getTargetRepository(RepositoryMetadata metadata) {
            return new CustomQueryDslJpaRepositoryImpl<>(getEntityInformation(metadata.getDomainType()), entityManager);
        }

        protected Class<?> getRepositoryBaseClass(RepositoryMetadata metadata) {
            return CustomQueryDslJpaRepository.class;
        }
    }
}

Step 4: Use the custom repository factory

Using annotation

@EnableJpaRepositories(repositoryFactoryBeanClass=CustomQueryDslJpaRepositoryFactoryBean.class)

OR using XML

<repositories base-package="com.acme.repository"  factory-class="com.acme.CustomQueryDslJpaRepositoryFactoryBean" />

Note: Don't place custom repository interface and implementation in the same directory as base-package. If you are placing then exclude them from scanning otherwise spring will try to create beans for them

Sample usage

public interface UserDemoRepository extends CustomQueryDslJpaRepository<UserDemo, Long>{
}

public class UserDemoService {
    @Inject 
    UserDemoRepository userDemoRepository;

    public Page<User> findAll(UserSearchCriteria userSearchCriteria, Pageable pageable) {
        QUserDemo user = QUserDemo.userDemo;
        return userDemoRepository.findAll(Projections.bean(UserDemo.class, user.id, user.username), UserPredicate.defaultUserSearch(userSearchCriteria), pageable);
    }

}

For more recent versions of Spring Data, I couldn't get the accepted answer to work without hitting issues, but found that going down the route from the Spring Data docs, does work by revising that answer as follows:

1. The repository interface

@NoRepositoryBean
public interface QueryDslPredicateAndProjectionExecutor<T, ID extends Serializable>
        extends JpaRepository<T, ID>, QueryDslPredicateExecutor<T> {

    <PROJ> Page<PROJ> customFindWithProjection(FactoryExpression<PROJ> factoryExpression, Predicate predicate, Pageable pageable);
}

2. The repository implementation

public class QueryDslJpaEnhancedRepositoryImpl<T, ID extends Serializable> extends QueryDslJpaRepository<T, ID>
        implements QueryDslPredicateAndProjectionExecutor<T, ID> {

    //All instance variables are available in super, but they are private
    private static final EntityPathResolver DEFAULT_ENTITY_PATH_RESOLVER = SimpleEntityPathResolver.INSTANCE;

    private final EntityPath<T> path;
    private final PathBuilder<T> builder;
    private final Querydsl querydsl;

    public QueryDslJpaEnhancedRepositoryImpl(JpaEntityInformation<T, ID> entityInformation, EntityManager entityManager) {
        this(entityInformation, entityManager, DEFAULT_ENTITY_PATH_RESOLVER);
    }

    public QueryDslJpaEnhancedRepositoryImpl(JpaEntityInformation<T, ID> entityInformation, EntityManager entityManager,
                                 EntityPathResolver resolver) {

        super(entityInformation, entityManager, resolver);
        this.path = resolver.createPath(entityInformation.getJavaType());
        this.builder = new PathBuilder<T>(path.getType(), path.getMetadata());
        this.querydsl = new Querydsl(entityManager, builder);
    }

    @Override
    public <PROJ> Page<PROJ> customFindWithProjection(FactoryExpression<PROJ> factoryExpression, Predicate predicate, Pageable pageable) {
        JPQLQuery countQuery = createQuery(predicate);
        JPQLQuery query = querydsl.applyPagination(pageable, createQuery(predicate));

        Long total = countQuery.count();
        List<PROJ> content = total > pageable.getOffset() ? query.list(factoryExpression) : Collections.<PROJ>emptyList();

        return new PageImpl<PROJ>(content, pageable, total);
    }
}

3. Setting the default repository implementation

@EnableJpaRepositories(
    repositoryBaseClass=QueryDslJpaEnhancedRepositoryImpl.class,
    basePackageClasses=SomeRepository.class)

For the current versions of Spring Data (1.11.1) and QueryDSL (4), you have to change the customFindWithProjection method implementation like this:

@Override
public <PROJ> Page<PROJ> customFindWithProjection(FactoryExpression<PROJ> factoryExpression, Predicate predicate, Pageable pageable) {

    final JPQLQuery<?> countQuery = createCountQuery(predicate);
    JPQLQuery<PROJ> query = querydsl.applyPagination(pageable, createQuery(predicate).select(factoryExpression));

    long total = countQuery.fetchCount();
    List<PROJ> content = pageable == null || total > pageable.getOffset() ? query.fetch() : Collections.<PROJ> emptyList();

    return new PageImpl<PROJ>(content, pageable, total);
}

The rest of the code remains the same.

As a (albeit very ugly and inefficient) workaround, I simply retrieved a regular Page containing the entities from my repository and manually mapped them to a projection in the controller like so:

@GetMapping(value = "/columns")
public Page<ColumnProjection> getColumns(@QuerydslPredicate(root = Column.class) final Predicate predicate,
                                         final Pageable pageable) {
 Page<Column> filteredColumns = columnRepository.findAll(predicate, pageable);
 List<ColumnProjection> filteredColumnProjections = new ArrayList<>();
 filteredColumns.forEach(c -> filteredColumnProjections.add(new ColumnProjectionImpl(c)));
 return new PageImpl<>(filteredColumnProjections, pageable, filteredColumnProjections.size());
}

Where ColumnProjectionImpl is a class implementing my ColumnProjection interface.

This was the easiest solution I could come up with while not having to adapt my existing ColumnRepository.