Spring Data REST-如何在投影中包含计算的数据? [英] Spring Data REST - How to include calculated data in a projection?

问题描述

我定义了以下域类.

贷款类别

@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;
}

津贴等级

@Data
@Entity
public class Allowance {

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

    @ManyToOne
    private AllowanceType allowanceType;


    private Double allowanceAmount;

}

我还为贷款类定义了一个投影界面，如下所示:

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

    String getLoanTitle();

    List<AllowanceProjection> getAllowances();

}

现在，我要在投影中包括贷款的总金额(通过对津贴"列表进行迭代来计算)，并将其发送到UI.在Spring Data REST中可以做到这一点吗?

解决方案

来自

并像这样使用Projection:

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

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

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

然后您可以获得全部金额的贷款:

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

---

一切看起来不错，但是这里有一个小"问题-对于结果中的每条记录，我们都会向数据库额外查询以计算总金额.因此，您在这里面临' N + 1个查询问题 `.

我对带有Projection的SDR中此问题的调查，可以在此处找到.

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?

解决方案

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.

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