Spring Data REST - How to include calculated data in a projection?

Question

I have the following domain classes defined.

Loan Class

@Data
@Entity
public class Loan {

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

    private String loanTitle;


    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    @JoinColumn(name = "loan_id")
    private List<Allowance> allowances;
}

Allowance class

@Data
@Entity
public class Allowance {

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

    @ManyToOne
    private AllowanceType allowanceType;


    private Double allowanceAmount;

}

I also have a projection interface defined for the loan class as follows:

@Projection(name = "studyLoanSingle", types = {Loan.class})
public interface LoanProjection {

    String getLoanTitle();

    List<AllowanceProjection> getAllowances();

}

Now I want to include the total amount of the loan(which is calculated by iterating the list of Allowances) in the projection and send it to the UI. Is it possible to do this in Spring Data REST?

Answer 1

From here:

You can annotate exposed properties in Projection with @Value using SpEL expressions to expose synthetic properties. Even invoke methods on other Spring beans and hand over the target to it for use in advanced calculations.

So you have to create a LoanRepo bean method (for example) that calculate the total amount of the given loan:

@Query("select sum(a.allowanceAmount) as amount from Loan l join l.allowances a where l = ?1")
Double getTotalAmountByLoan(Loan loan);

and use like this Projection:

@Projection(name = "totalAmount", types = Loan.class)
public interface LoanTotalAmount {

    @Value("#{target}")
    Loan getLoan();

    @Value("#{@loanRepo.getTotalAmountByLoan(target)}")    
    Double getAmount();
}

Then you can get your loans with total amount:

GET http://localhost:8080/api/loans?projection=totalAmount

---

All looks fine but we have a 'small' issue here - for each record in the result we get an extra query to the DB that calculate total amount. So you faces here with 'N+1 queries issue`.

My investigation of this problem in SDR with Projections you can find here.

Answer 2

Place on domain objects methods to solve a view representation (projection) is not the best solution.

Place on repository would be useful for simple use cases, for complex issues where Java 8 will be present thanks to default methods of interface you can use this simple trick.

@Projection(name = "studyLoanSingle", types = Loan.class)
public interface LoanProjection {

    String getLoanTitle();

    //If no need Allowances on json
    @JsonIgnore
    List<Allowance> getAllowances();

    public default Double getAmount() {
        Double result = new Double(0);
        for (Allowance a : getAllowances()) {
           result += a.getAllowanceAmount();            
        }
        return result;
    }
}