JPA OneToMany 关系中按子实体属性过滤集合的策略与实践

// Class A.java
import javax.persistence.*;
import java.util.List;

@Entity
public class A {
    @Id
    @GeneratedValue
    private Long id;
    private String name;

    @OneToMany(mappedBy = "a", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<B> b;

    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public List<B> getB() { return b; }
    public void setB(List<B> b) { this.b = b; }
}

// Class B.java
import javax.persistence.*;

@Entity
public class B {
    @Id
    @GeneratedValue
    private Long id;
    private String property1;
    private String property2;

    @ManyToOne
    private A a;

    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getProperty1() { return property1; }
    public void setProperty1(String property1) { this.property1 = property1; }
    public String getProperty2() { return property2; }
    public void setProperty2(String property2) { this.property2 = property2; }
    public A getA() { return a; }
    public void setA(A a) { this.a = a; }
}

import com.blazebit.persistence.view.EntityView;
import com.blazebit.persistence.view.IdMapping;
import com.blazebit.persistence.view.Mapping;
import java.util.Set; // 使用Set而非List，避免重复，且通常更符合业务逻辑

@EntityView(A.class)
public interface ADto {
    @IdMapping
    Long getId();
    String getName();

    // 关键：使用@Mapping注解结合JPQL表达式来过滤子集合
    // "b[property1 = 'ABC']" 表示只选择那些 property1 等于 'ABC' 的 B 对象
    @Mapping("b[property1 = 'ABC']")
    Set<BDto> getB();

    @EntityView(B.class)
    interface BDto {
        @IdMapping
        Long getId();
        String getProperty2();
        // 如果需要，也可以包含 property1
        // String getProperty1();
    }
}

import com.blazebit.persistence.CriteriaBuilderFactory;
import com.blazebit.persistence.view.EntityViewManager;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

// 假设已经通过依赖注入获取了 EntityManager 和 EntityViewManager
@PersistenceContext
private EntityManager entityManager;

// EntityViewManager 通常通过配置注入
// private EntityViewManager entityViewManager; 

public ADto findAWithFilteredB(Long id) {
    // 最简单的查询方式：通过ID查找
    ADto aDto = entityViewManager.find(entityManager, ADto.class, id);
    return aDto;
}

// 也可以结合Criteria API或Spring Data进行更复杂的查询
// 例如，查询所有A，并分页
// Page<ADto> findAll(Pageable pageable); // 需要Spring Data集成

import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
import org.springframework.data.jpa.repository.JpaRepository;
import com.blazebit.persistence.spring.data.repository.EntityViewRepository; // 或直接 JpaRepository

// 假设 ADto 已经定义
public interface ARepository extends JpaRepository<A, Long>, EntityViewRepository<ADto, Long> {
    Page<ADto> findAll(Pageable pageable);
    // 还可以定义其他查询方法，返回 ADto
    // ADto findByName(String name);
}