关于mybatis中collection一对多关联查询分页出错问题总结
在使用mybatis作为数据分页查询的时候，如果使用了collection进行一对多关联的时候有可能出现分页不正确的问题：
以下是两个实体类，老师类 和班级类，一个老师可以对应多个班级（暂假设为为一对多的关系）
1.老师类
import java.util.List; /*老师*/ @Data @Getter @Setter public class Teacher { private Long id; private String name; private Integer age; private List<Group> groupList; }
数据库数据：
1 张小花老师 24
2 李戴莫老师 26
3 王敖明老师 55
4 周小玲老师 45
5 周涛老师 33
2.班级类
/*班级*/ @Data @Getter @Setter public class Group { private Long id; private String name; private Integer number; //private Liststudent; }
数据库数据：
3 花桥河高一(4)班 44 2
4 花桥河高一(5)班 38 3
5 花桥河高一(6)班 38 4
6 花桥河高一(7)班 38 1
7 花桥河高一(8)班 38 1
8 花桥河高一(8)班 38 5
TeacherMapper.xml类
<mapper namespace="com.oasis.test.mapper.TeacherMapper"> <resultMap id="teacherResultMap" type="com.oasis.test.entity.Teacher"> <id column="tid" property="id"/> <result column="teacher_name" property="name"/> <result column="age" property="age"/> <collection property="groupList" ofType="com.oasis.test.entity.Group" column="id"> <id column="gid" property="id"/> <result column="group_name" property="name"/> <result column="number" property="number"/> collection> resultMap> <select id="getTeacherList" parameterType="java.lang.Long" resultMap="teacherResultMap"> SELECT t.ID tid, t.TEACHER_NAME,AGE,g.GROUP_NAME,NUMBER,g.id gid FROM t_teacher t LEFT JOIN t_group g ON t.id=g.teacher_id limit 0,5 select>
查询结果：
"data": [
{
"id": 2,
"name": "李戴莫老师",
"age": 26,
"groupList": [
{
"id": 3,
"name": "花桥河高一(4)班",
"number": 44
}
]
},
{
"id": 3,
"name": "王敖明老师",
"age": 55,
"groupList": [
{
"id": 4,
"name": "花桥河高一(5)班",
"number": 38
}
]
},
{
"id": 4,
"name": "周小玲老师",
"age": 45,
"groupList": [
{
"id": 5,
"name": "花桥河高一(6)班",
"number": 38
}
]
},
{
"id": 1,
"name": "张小花老师",
"age": 24,
"groupList": [
{
"id": 6,
"name": "花桥河高一(7)班",
"number": 38
},
{
"id": 7,
"name": "花桥河高一(8)班",
"number": 38
}
]
}
]
从查询结果看只有4条数据，但是我查询的是5条数据（见mapper绿色部分），且数据库数据条数正好有五条，出现这个问题的主要原因是collection采用以上写法，他会将collection中查询到的条数作为分页约束（见以上金色部分）
正确写法：实体类和数据库数据不变，在collection中添加select关联查询：
1.修改TeacherMapper的collection：
<mapper namespace="com.oasis.test.mapper.TeacherMapper"> <resultMap id="teacherResultMap" type="com.oasis.test.entity.Teacher"> <id column="id" property="id"/> <result column="teacher_name" property="name"/> <result column="age" property="age"/> <collection property="groupList" ofType="com.oasis.test.entity.Group" column="id" select="com.oasis.test.mapper.GroupMapper.getGroupListByTeacherId"> collection> resultMap> <select id="getTeacherList" parameterType="java.lang.Long" resultMap="teacherResultMap"> SELECT ID,TEACHER_NAME,AGE FROM t_teacher limit 0,5 select>
2.添加GroupMapper.xml类
<mapper namespace="com.oasis.test.mapper.GroupMapper"> <resultMap id="groupResultMap" type="com.oasis.test.entity.Group"> <id column="id" property="id"/> <result column="group_name" property="name"/> <result column="number" property="number"/> resultMap> <select id="getGroupListByTeacherId" parameterType="java.lang.Long" resultMap="groupResultMap"> SELECT ID,GROUP_NAME,NUMBER FROM t_group WHERE teacher_id=#{id} select> mapper>
3.执行命令得到查询结果
"data": [
{
"id": 1,
"name": "张小花老师",
"age": 24,
"groupList": [
{
"id": 6,
"name": "花桥河高一(7)班",
"number": 38
},
{
"id": 7,
"name": "花桥河高一(8)班",
"number": 38
}
]
},
{
"id": 2,
"name": "李戴莫老师",
"age": 26,
"groupList": [
{
"id": 3,
"name": "花桥河高一(4)班",
"number": 44
}
]
},
{
"id": 3,
"name": "王敖明老师",
"age": 55,
"groupList": [
{
"id": 4,
"name": "花桥河高一(5)班",
"number": 38
}
]
},
{
"id": 4,
"name": "周小玲老师",
"age": 45,
"groupList": [
{
"id": 5,
"name": "花桥河高一(6)班",
"number": 38
}
]
},
{
"id": 5,
"name": "周涛老师",
"age": 33,
"groupList": [
{
"id": 8,
"name": "花桥河高一(8)班",
"number": 38
}
]
}
]
总共5条数据，符合要求
以上是开发中的一些总结，记录下来，如有错误之处请指出