Failed to convert from type [java.lang.Object[]] to type

Question

I have spring web app (JPA/Hibernate + MySQL). I have two DAO classes.

CustomerDAO

@Entity
@Table(name = "customers")
public class Customer {

  @Id
  @Column(name = "customer_id")
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long id;

  @Column(name = "name", length = 50)
  private String name;

  @Column(name = "surname", length = 50)
  private String surname;

  @OneToMany(mappedBy = "customer")
  private Set<Order> orders = new HashSet<>();
}

OrderDAO

@Entity
@Table(name = "orders")
public class Order {

  @Id
  @Column(name = "order_id")
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long id;

  @Column(name = "date")
  private Date date;

  @Digits(integer = 5, fraction = 2)
  @Column(name = "amount")
  private BigDecimal amount;

  @ManyToOne(fetch = FetchType.LAZY)
  @JoinColumn(name = "customer_id")
  private Customer customer;

  @OneToMany(mappedBy = "order")
  private Set<OrderDetail> ordersDetails = new HashSet<>();

And i have a class for retrieving data from DB:

@Repository
public interface OrderDAO extends JpaRepository<Order, Long> {

    @Query("select o.customer.surname, sum(o.amount) as s from Order as o group by o.customer")
    List<Customer> findCustomersBySumOfAmount();

}

It is giving me result like this:

+---------+---------------+
| surname | sum of amount | 
+---------+---------------+
|Bielecki | 141.49        | 
|Bielen   | 130.34        |
......

Now i want 'unbox' data from DB with this method List<Customer> findCustomersBySumOfAmount()

I have method for this in my spring controller class:

 List<Customer> findCustomersBySumOfAmount = orderService.findCustomersBySumOfAmount();
 model.addAttribute("findCustomersBySumOfAmount", findCustomersBySumOfAmount);

 for(Customer c : findCustomersBySumOfAmount) {
     String s = c.getSurname();
     System.out.println(c);
 }

And i have error:

Failed to convert from type [java.lang.Object[]] to type [com.twistezo.models.Customer] for value '{Bielecki, 141.49}'; nested exception is org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.String] to type [com.twistezo.models.Customer]

I suppose it's because I'm getting List<Object[]>. I know that I can iterate between this List<Object[]> of my data but maybe there is some simpler way to retrieve data directly to <Customer> ? I'm new in this stuff. Since now I used methods like List<Customer> findAll() without @Query annotation and i'm looking for similar "unboxing".

I was trying do something like this (add Customer.class in query) without effect:

@Query("select o.customer.surname, sum(o.amount) as s from Order as o group by o.customer", Customer.class) List<Customer> findCustomersBySumOfAmount();

Answer 1

I already had this error message. In my case, I was doing something like the code below:

@Repository
public interface RepositoryA extends CrudRepository<EntityA, Long> {

  @Query(nativeQuery = true, value = " ...SQL_1... ")
  List<EntityA> getListOfEntityA(...PARAMS_1...);

  @Query(nativeQuery = true, value = " ...SQL_2... ")
  List<EntityB> getListOfEntityB(...PARAMS_2...);

}

I was parameterizing the repository interface with an "EntityA" and had an other method returning other type parametrized with an "EntityB".

In your case you are parameterizing the JpaRepository with "Order" and then using a method that returns a list of "Customer"s... Maybe this is causing this exception.

Answer 2

I would suggest creating a POJO class just to store the results of that query:

package com.mypackage;

public class CustomerAmountResult{

    private String surname;
    private BigDecimal amountSum;

    public CustomerAmountResult(String surname, BigDecimal amountSum){
       this.surname = surname;
       this.amountSum = amountSum;
    }

    // getters / setters
}

Then change your query to the following:

@Query("select NEW com.mypackage.CustomerAmountResult(
            o.customer.surname, sum(o.amount)) 
        from Order as o
        group by o.customer.surname") 
List<CustomerAmountResult> findCustomersBySumOfAmount();

Thanks to that you will not need to parse the result set manually.