Convert native (join) queries in jpa (Hibernate) to json

Question

I'm working on a spring boot project and am using jpa for data persistence. Right now I have two tables that are related to each other, users and items. A user can own any number of items while an item can only owned by one user.
Here are my pojos:
Users

@Entity
@Table(name="users", uniqueConstraints = {
            @UniqueConstraint(columnNames = {
                "email"
            })
    })
@EntityListeners(AuditingEntityListener.class)
@JsonIgnoreProperties(value = {"createdAt", "updatedAt"}, allowGetters = true)
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotBlank
    @NaturalId
    @Column(unique = true)
    private String email;

    @NotBlank
    @JsonIgnore
    private String password;

    @NotBlank
    private String first_name;

    @NotBlank
    private String last_name;

    @OneToMany(cascade = CascadeType.REMOVE, orphanRemoval = true)
    @JsonIgnore
    private Set<Item> items;

    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "user_roles",
            joinColumns = @JoinColumn(name = "user_id"),
            inverseJoinColumns = @JoinColumn(name = "role_id"))
    private Set<Role> roles = new HashSet<>();


    @Column(nullable = false, updatable = false)
    @Temporal(TemporalType.TIMESTAMP)
    @CreatedDate
    private Date createdAt;

    @Column(nullable = false)
    @Temporal(TemporalType.TIMESTAMP)
    @LastModifiedDate
    private Date updatedAt;

Items

@Entity
@Table(name="items")
@EntityListeners(AuditingEntityListener.class)
@JsonIgnoreProperties(value = {"createdAt", "updatedAt"}, allowGetters = true)
public class Item {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotBlank
    @NotNull
    private String name;

    private String description;

    @NotNull 
    private Date purchase_date;

    private double price;

    @ManyToOne(fetch = FetchType.LAZY, optional = true)
    @JoinColumn(name = "owner", nullable = true)
    private User owner;

    @Column(nullable = false, updatable = false)
    @Temporal(TemporalType.TIMESTAMP)
    @CreatedDate
    private Date createdAt;

    @Column(nullable = false)
    @Temporal(TemporalType.TIMESTAMP)
    @LastModifiedDate
    private Date updatedAt;

Now I want to get all items as RESTfull JSONs. I need to join users with items so that I could get the following JSON:

{
    "item_id":1,
    "name":"item1",
    "price":120.00,

    etc .....

    "owner_id":1,
    "owner_name":"Jon"
    etc ....

}

So I'm using a custom native query

SELECT i.id, i.name, i.description ...... u.id, u.name ..... from items i , users u where i.owner = u.id

And then I return query.getResultList() however this returns and array of strings rather than a json like that

[

    [ 1 , "item1" , 120.00 , ..... , 1 , "Jon" , ....]
    [ 2 , "item2" , 420.00 ....   ]
etc...
]

How can I cast the returned object to either a JSON directly or to a list of maps that maps the column name to the value and then convert that to JSON?

Answer 1

You could use the constructor expression to create a DTO (Data Transfer Object) that contains the data you need.

package dto;

public class ItemDTO {

   private final Long item_id;
   private final String name;
   private final Long owner_id;
   private final String owner_name;

   public ItemDTO(Long item_id, String name, Long owner_id, String owner_name) {
      // set the fields
   }

   // getters
}

And then use this DTO in a constructor expression query (important note: this only works with a JPQL query not a native query)

SELECT new dto.ItemDTO(i.id, i.name, i.owner.id, i.owner.name) from Item i where i.owner.id = u.id

This DTO can the be used to serialize to JSON.

Read more about the construtor expression here: https://vladmihalcea.com/the-best-way-to-map-a-projection-query-to-a-dto-with-jpa-and-hibernate/

Answer 2

You can use this, there is no need for new DTO on every request.

@SuppressWarnings("unchecked")
public static List<ObjectNode> getQueryResult(EntityManager entityManager, String nativeQuery, Object... parameters) {

    Query localQuery = entityManager.createNativeQuery(nativeQuery,Tuple.class);

    for(int i = 0; i < parameters.length; i++)
        localQuery.setParameter(i+1, parameters[i]);

    return toJson(localQuery.getResultList());
}

private static List<ObjectNode> toJson(List<Tuple> results) {

    List<ObjectNode> json = new ArrayList<>();

    ObjectMapper mapper = new ObjectMapper();

    for (Tuple tuple : results)
    {
        List<TupleElement<?>> cols = tuple.getElements();

        ObjectNode node = mapper.createObjectNode();

        for (TupleElement col : cols)
            node.put(col.getAlias(), tuple.get(col.getAlias()).toString());

        json.add(node);
    }
    return json;
}