今天在写一个接口时，有两级目录（父子关系），接口需要把两级数据以嵌套的形式返回给前端。我这个新手菜鸟一上来就查询两次sql，然后业务中处理嵌套关系，事实这种方法也能达到目的。但主管PR代码时，问为啥不在sql语句中处理呢？我当然不知道还有这种骚操作，下面是在主管的指导下改造的通过sql语句实现的，记录下来慢慢欣赏。
1、先理一下表中的数据结构
①project_case表，其中有个数组类型字段category_id_list，用来记录此条数据对应的二级目录
②project_case_category表，其中parent_id为0表示一级目录，其他则为二级目录
2、sql中实现代码
<resultMap id="ProjectCaseCategoryVo" type="com.hikvision.idatafusion.dhidata.bean.vo.projectcase.ProjectCaseCategoryVo">
<result column="first_category_id" jdbcType="BIGINT" property="id"/>
<result column="first_category_name" jdbcType="VARCHAR" property="categoryName"/>
<collection property="children" ofType="com.hikvision.idatafusion.dhidata.bean.vo.projectcase.ProjectCaseCategoryVo" javaType="java.util.List">
<result property="id" column="second_category_id" jdbcType="INTEGER"/>
<result property="categoryName" column="second_category_name" jdbcType="VARCHAR"/>
</collection>
</resultMap>
<select id="getListFront" resultMap="ProjectCaseCategoryVo">
select first_category.id as first_category_id, first_category.category_name as first_category_name,
second_category.id as second_category_id, second_category.category_name as second_category_name
from (
select distinct unnest(category_id_list) as category_id from project_case where delete_flag = 0 and publish_state = '1'
) t
left join project_case_category second_category on second_category.id = t.category_id
left join project_case_category first_category on first_category.id = second_category.parent_id
where second_category.delete_flag = 0 and first_category.delete_flag = 0
order by first_category.category_sort desc, second_category.category_sort desc
</select>上面的sql写的时候要注意两点：①property要和实体类中的属性对应，比如id、categoryName、children都是实体类中的字段；②column要和数据库中的字段保持一致，比如first_category_id、first_category_name等，都是sql语句查询出来后重命名的字段。
3、sql中对应的实体类代码
①ProjectCaseCategoryVo
@Data
@ToString
@NoArgsConstructor
@EqualsAndHashCode(callSuper = false)
public class ProjectCaseCategoryVo extends ProjectCaseCategory implements Serializable {
private static final long serialVersionUID = 1L;
@ApiModelProperty(value = "同一类型的案例")
private List<ProjectCaseCategory> projectCaseVoList;
@ApiModelProperty(value = "子分类")
private List<ProjectCaseCategoryVo> children = new ArrayList<>();
}②ProjectCaseCategory
@Data
@ToString
@NoArgsConstructor
@EqualsAndHashCode(callSuper = false)
public class ProjectCaseCategory extends AbstractEntity {
private static final long serialVersionUID = 1L;
@ApiModelProperty("分类名称")
private String categoryName;
@ApiModelProperty("分类排序")
private Integer categorySort;
@ApiModelProperty(value = "父节点ID")
public Long parentId;
}4、sql中处理后，业务层的代码就简单了，查询后直接返回给前端便可
List<ProjectCaseCategory> list = projectCaseCategoryDao.getListFront(dto);
List<ProjectCaseCategoryVo> projectCaseCategoryVos = BeanUtil.copyProperties(list, ProjectCaseCategoryVo.class);
return projectCaseCategoryVos;5、此时返回给前端的数据格式正是想要的
6、业务代码中有个工具类BeanUtil，是在实体类转换工具BeanUtils基础上封装的
public class BeanUtil extends BeanUtils {
/**
* copy 对象
*
* @param sourceList
* @param targetClazz
* @param <T>
* @return
*/
public static <T> List<T> copyProperties(List<?> sourceList, Class<T> targetClazz) {
List<T> resultList = new ArrayList<T>();
if (CollectionUtils.isEmpty(sourceList)) {
return resultList;
}
for (Object object : sourceList) {
try {
T target = targetClazz.newInstance();
copyProperties(object, target);
resultList.add(target);
} catch (Exception e) {
log.error("copyProperties fail", e);
throw new BusinessException(ErrorCodes.OBJECT_CLONE_FAIL);
}
}
return resultList;
}
/**
* copy 对象
*
* @param source
* @param targetClazz
* @param <T>
* @return
*/
public static <T> T copyProperties(Object source, Class<T> targetClazz) {
if (source == null) {
return null;
}
T target = null;
try {
target = targetClazz.newInstance();
copyProperties(source, target);
} catch (Exception e) {
log.error("copyProperties fail", e);
throw new BusinessException(ErrorCodes.OBJECT_CLONE_FAIL);
}
return target;
}
/**
* 判断对象的所有属性是否都为空
* @param object
* @return
* @throws IllegalAccessException
*/
public static boolean checkFieldAllNull(Object object, List<String> ignoreProperties) {
for (Field f : object.getClass().getDeclaredFields()) {
if (ignoreProperties != null && ignoreProperties.contains(f.getName())) {
continue;
}
f.setAccessible(true);
if (Modifier.isFinal(f.getModifiers()) && Modifier.isStatic(f.getModifiers())) {
continue;
}
try {
if (!isEmpty(f.get(object))) {
System.out.println(f.getName());
return false;
}
} catch (Exception e) {
return false;
}
f.setAccessible(false);
}
//父类public属性
for (Field f : object.getClass().getFields()) {
if (ignoreProperties != null && ignoreProperties.contains(f.getName())) {
continue;
}
f.setAccessible(true);
if (Modifier.isFinal(f.getModifiers()) && Modifier.isStatic(f.getModifiers())) {
continue;
}
try {
if (!isEmpty(f.get(object))) {
return false;
}
} catch (Exception e) {
return false;
}
f.setAccessible(false);
}
return true;
}
@SuppressWarnings("rawtypes")
private static boolean isEmpty(Object object) {
if (object == null) {
return true;
} else if (object instanceof String && (object.toString().equals(""))) {
return true;
} else if (object instanceof Collection && ((Collection) object).isEmpty()) {
return true;
} else if (object instanceof Map && ((Map) object).isEmpty()) {
return true;
} else if (object instanceof Object[] && ((Object[]) object).length == 0) {
return true;
}
return false;
}
}
