QueryDSL分组与复杂DTO投影实践指南

package com.example.technologyradar.model;

import com.example.technologyradar.dto.constant.TechnologyStatus;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;
import java.util.List;

@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Technology {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO, generator = "native")
    @GenericGenerator(name="native", strategy = "native")
    private Long id;
    private String name;
    @Enumerated(EnumType.STRING)
    private TechnologyStatus technologyStatus;
    // ... 其他关联字段，如Category, Coordinate, Projects
}

// TechnologyBasicDataDTO (假设包含id和name)
package com.example.technologyradar.dto;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class TechnologyBasicDataDTO {
    private Long id;
    private String name;
    // 根据实际需求添加Technology实体的其他基本字段
}

// TechnologyByStatusDTO
package com.example.technologyradar.dto;

import com.example.technologyradar.dto.constant.TechnologyStatus;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class TechnologyByStatusDTO {
    TechnologyStatus status;
    List<TechnologyBasicDataDTO> technologies;
}

// 错误示例：无法直接编译或实现预期效果
@Override
public List<TechnologyByStatusDTO> getTechnologyByStatus() {
    QTechnology technology = QTechnology.technology;
    // QCoordinate coordinate = QCoordinate.coordinate; // 如果不需要coordinate字段，可以省略join

    return jpaQueryFactory.from(technology)
            // .innerJoin(technology.coordinate, coordinate) // 如果不需要coordinate字段，可以省略join
            .groupBy(technology.technologyStatus)
            .select(Projections.constructor(TechnologyByStatusDTO.class,
                    technology.technologyStatus,
                    list(TechnologyBasicDataDTO.class))) // 这里的list(TechnologyBasicDataDTO.class)是无法编译的
            .fetch();
}

import com.querydsl.core.group.GroupBy;
import java.util.Map;
import java.util.List;
import java.util.stream.Collectors;

// ...
public List<TechnologyByStatusDTO> getTechnologyByStatus() {
    QTechnology technology = QTechnology.technology;

    // 步骤1: 使用 QueryDSL 的 GroupBy 转换器进行分组和初步聚合
    // 结果将是一个 Map，键为 technologyStatus，值为该状态下的 Technology 实体列表
    Map<TechnologyStatus, List<Technology>> groupedTechnologies = jpaQueryFactory
        .from(technology)
        // 如果查询中不需要用到 coordinate 字段，可以省略 innerJoin
        // .innerJoin(technology.coordinate, coordinate)
        .transform(GroupBy.groupBy(technology.technologyStatus)
            .as(GroupBy.list(technology))); // 将每个组的所有 Technology 实体收集成列表

    // 步骤2: 将 Map<TechnologyStatus, List<Technology>> 转换为目标 List<TechnologyByStatusDTO>
    return groupedTechnologies.entrySet().stream()
        .map(entry -> {
            TechnologyStatus status = entry.getKey();
            List<Technology> technologiesInGroup = entry.getValue();

            // 将 List<Technology> 映射为 List<TechnologyBasicDataDTO>
            List<TechnologyBasicDataDTO> basicTechDTOs = technologiesInGroup.stream()
                .map(t -> new TechnologyBasicDataDTO(t.getId(), t.getName())) // 假设 TechnologyBasicDataDTO 构造函数接受 id 和 name
                .collect(Collectors.toList());

            // 构建 TechnologyByStatusDTO
            return new TechnologyByStatusDTO(status, basicTechDTOs);
        })
        .collect(Collectors.toList());
}