02 Mybatis-Plus collection标签嵌套查询(select)的写法

1.问题描述：

前置：
在association标签中 javaType属性指向的是实体类的属性。
在collection标签中 javaType属性指向的是集合的类型 ofType指向的是集合的泛型类型 。
mybatis-plus selectPage与一对多查询时存在问题。
1.1实体类

@Data
@NoArgsConstructor
@AllArgsConstructor
@TableName("sys_class")
public class ClassInfo {

    @TableId(type = IdType.AUTO)
    private Integer id;
    private String grade;
    private String className;
    private String classNickname;
    private String ratedNum;

    @TableField(fill = FieldFill.INSERT)
    @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss",timezone = "GMT+8")
    private Date createTime;  //创建时间

    @TableField(fill = FieldFill.UPDATE)
    @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss",timezone = "GMT+8")
    private Date updateTime;  //更新时间

    @TableField(exist = false) // 属性字段在表中不存在
    private List<Student> studentList;      //该班级的学生
    @TableField(exist = false) // 属性字段在表中不存在
    private List<Teacher> teacherList;      //管理该班级的教师
}

@Data
@NoArgsConstructor
@AllArgsConstructor
@TableName("sys_student")
public class Student {

    @TableId(type = IdType.AUTO)
    private Integer id;
    private String name;
    private String username;
    private String password;
    private SexEnum sex;
    private Date birth;
    private Date schoolDate;
    private Integer classId;
    private String portrait; //头像

    @TableField(fill = FieldFill.INSERT)
    @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss", timezone = "GMT+8")
    private Date createTime;  //创建时间

    @TableField(fill = FieldFill.UPDATE)
    @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss", timezone = "GMT+8")
    private Date updateTime;  //更新时间

    @TableField(exist = false)// 属性字段在表中不存在
    private ClassInfo classInfo;//当前学生的班级
    @TableField(exist = false) // 属性字段在表中不存在
    private List<Family> familyList;  //当前学生的家长
}

1.2ClassInfoMapper.java

public interface ClassInfoMapper extends BaseMapper<ClassInfo> {
    //    一对多查询
    Page<ClassInfo> findPage(Page<ClassInfo> page, @Param("className") String className);

    //所有班级id
    @Select("select id  from sys_class")
    List<Integer> getClassIds();
}

1.3原mapper.xml如下：

  <resultMap id="classInfoMap" type="ClassInfo">
   --         as别名与column一致,column与mysql表字段一致，property与java实体一致
         <id property="id" column="id"/>
         <result property="grade" column="grade"/>
         <result property="className" column="class_name"/>
         <result property="classNickname" column="class_nickname"/>
         <result property="ratedNum" column="rated_num"/>
         <result property="createTime" column="create_time"/>
         <result property="updateTime" column="update_time"/>
         <collection property="studentList" javaType="ArrayList" ofType="Student">
             <id property="id" column="s_id"/>
             <result property="name" column="s_name"/>
             <result property="sex" column="s_sex"/>
         </collection>
     </resultMap>
     <select id="findPage" resultMap="classInfoMap">
         select c.*, s.id as s_id,s.name s_name,s.sex s_sex
         from sys_class c left join sys_student s
         on c.id =s.class_id
         where c.class_name like concat('%',#{className},'%')
     </select>

1.4原查询结果：

2.解决方案：

分两次sql查询：

 <resultMap id="classInfoMap" type="ClassInfo">
        <id property="id" column="id"/>
        <result property="grade" column="grade"/>
        <result property="className" column="class_name"/>
        <result property="classNickname" column="class_nickname"/>
        <result property="ratedNum" column="rated_num"/>
        <result property="createTime" column="create_time"/>
        <result property="updateTime" column="update_time"/>
        <collection property="studentList" ofType="Student"
                    select="queryStudentList" column="id">
            <id property="id" column="id"/>
            <result property="name" column="name"/>
            <result property="sex" column="sex"/>
        </collection>
    </resultMap>
    <!-- 主查询sql -->
    <select id="findPage" resultMap="classInfoMap">
--         as别名与column一致,column与mysql表字段一致，property与java实体一致
         select c.*
         from sys_class c
         where c.class_name like concat('%',#{className},'%')
    </select>
    <!-- 子查询sql -->
    <select id="queryStudentList" resultType="Student">
        select s.id,s.name,s.sex
        from sys_student s
        where s.class_id=#{id}
    </select>

附：多对多查询
表结构：sys_student，sys_family，中间表sys_student_family

    <!--    学生-家长列表展示-->
    <resultMap id="Student_familyMap" type="Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="sex" column="sex"/>
        <result property="portrait" column="portrait"/>
        <collection property="familyList" ofType="Family"
                    select="queryFamilyList" column="id">
            <id property="id" column="id"/>
            <result property="name" column="name"/>
            <result property="sex" column="sex"/>
            <result property="portrait" column="portrait"/>
        </collection>
    </resultMap>
    <!-- 主查询sql -->
    <select id="showStudentsAndFamily" resultMap="Student_familyMap">
         select s.id,s.name,s.sex,s.portrait
         from sys_student s
         where s.name like concat('%',#{name},'%')
    </select>
    <!-- 子查询sql -->
    <select id="queryFamilyList" resultType="Family">
        select f.id,f.name,f.sex,f.portrait
        from sys_family f join sys_student_family sf
        where f.id = sf.family_id
		and #{id} = sf.student_id
    </select>

查询结果：