Custom Result Set Mapping in Spring Data JPA: Native Query to DTO Mapping Guide

In Spring Data JPA, when working with native queries, the default result mapping may not be sufficient if the query returns a result that doesn't map directly to an entity or needs additional customization. For this purpose, you can use Result Set Mapping to create custom mappings for native query results.

Below is a step-by-step guide to implement Result Set Mapping in Spring Data JPA from scratch:

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1. Use @SqlResultSetMapping for Custom Mapping

You can define a custom result set mapping using the @SqlResultSetMapping annotation. This can be used to map query results to either:

• Entities
• Scalar values
• DTOs

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2. Define the Entity and DTO Classes

Let's assume you have an Employee entity and want to map a custom result (e.g., only name and salary) into a DTO.

Entity Class

@Entity
@Table(name = "employees")
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

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

    @Column(name = "salary")
    private Double salary;

    // Getters and Setters
}

DTO Class

public class EmployeeDTO {
    private String name;
    private Double salary;

    public EmployeeDTO(String name, Double salary) {
        this.name = name;
        this.salary = salary;
    }

    // Getters and Setters
}

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3. Define Custom Mapping Using @SqlResultSetMapping

You define custom mappings at the entity level using @SqlResultSetMapping. This is typically done for scalar values or DTOs.

Add @SqlResultSetMapping to the Employee entity:

@Entity
@Table(name = "employees")
@SqlResultSetMapping(
    name = "EmployeeDTOResult",
    classes = @ConstructorResult(
        targetClass = EmployeeDTO.class,
        columns = {
            @ColumnResult(name = "name", type = String.class),
            @ColumnResult(name = "salary", type = Double.class)
        }
    )
)
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

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

    @Column(name = "salary")
    private Double salary;

    // Getters and Setters
}

• @SqlResultSetMapping: Defines the mapping for the native query results.
• @ConstructorResult: Maps query results to the constructor of the EmployeeDTO.
• @ColumnResult: Maps individual columns from the result set to parameters in the constructor.

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4. Write the Native Query

Use the @NamedNativeQuery annotation or Spring Data JPA repository to execute the query.

Add the @NamedNativeQuery:

@NamedNativeQuery(
    name = "Employee.findEmployeeDTO",
    query = "SELECT e.name, e.salary FROM employees e WHERE e.salary > :minSalary",
    resultSetMapping = "EmployeeDTOResult"
)
@Entity
public class Employee {
    // Existing fields and mappings
}

• @NamedNativeQuery: Defines the query and the mapping to be used.
• resultSetMapping: Specifies the @SqlResultSetMapping name.

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5. Execute the Query Using the Repository

Create a repository to execute the query.

Repository Interface

@Repository
public interface EmployeeRepository extends JpaRepository<Employee, Long> {
    @Query(nativeQuery = true, name = "Employee.findEmployeeDTO")
    List<EmployeeDTO> findEmployeeDTO(@Param("minSalary") Double minSalary);
}

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6. Test the Implementation

You can now test the implementation by calling the repository method.

Test Code

@Service
public class EmployeeService {
    @Autowired
    private EmployeeRepository employeeRepository;

    public List<EmployeeDTO> getEmployeesWithHighSalary(double minSalary) {
        return employeeRepository.findEmployeeDTO(minSalary);
    }
}

@RestController
@RequestMapping("/employees")
public class EmployeeController {
    @Autowired
    private EmployeeService employeeService;

    @GetMapping("/high-salary")
    public List<EmployeeDTO> getHighSalaryEmployees(@RequestParam double minSalary) {
        return employeeService.getEmployeesWithHighSalary(minSalary);
    }
}

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Summary of Annotations

• @SqlResultSetMapping: Custom mapping for the query results.
• @ColumnResult: Maps columns to constructor parameters or fields.
• @ConstructorResult: Maps the result to a DTO.
• @NamedNativeQuery: Defines the native query and its mapping.