mybatis一对多关联关系

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　　在《mybatis一对一关联关系》中，我们以班级和教师的例子介绍了一对一的关联关系，现在引入学生类，一个学生对应一个班级，一个班级对应多个学生，来介绍mybatis如何处理一对多的关联关系。
　　
　　首先创建表和实体类：

teacher表：

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CREATE TABLE teacher(
	t_id INT PRIMARY KEY AUTO_INCREMENT, 
	t_name VARCHAR(20)
);
INSERT INTO teacher(t_name) VALUES('LS1');
INSERT INTO teacher(t_name) VALUES('LS2');

image_1b4d974o2lm7dfiser8qvj159.png-13.4kB

class表：

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CREATE TABLE class(
	c_id INT PRIMARY KEY AUTO_INCREMENT, 
	c_name VARCHAR(20), 
	teacher_id INT
);
ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);	
INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1);
INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);

image_1b4d4baab1lar1uf61hfj2so1ge613.png-14kB

其中teacher_id是指向teacher表的外键：

image_1b4d98u561fcn30bon314mp1k6vm.png-23.3kB

student表：

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CREATE TABLE student(
	s_id INT PRIMARY KEY AUTO_INCREMENT, 
	s_name VARCHAR(20), 
	class_id INT
);
ALTER TABLE student ADD CONSTRAINT fk_class_id FOREIGN KEY (class_id) REFERENCES class(c_id);   
INSERT INTO student(s_name, class_id) VALUES('xs_A', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_B', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_C', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_D', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_E', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);

image_1b4d340jksb31n4511659qhbql9.png-12.2kB

其中class_id是指向class表的外键：

image_1b4d3uqo3hmueec1c961m57e7cm.png-17.6kB

实体类Student，Teacher和Classes：

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public class Student {
	private int id;
	private String name;
    //getters and setters
    
	@Override
	public String toString() {
		return "Student [id=" + id + ", name=" + name + "]";
	}
	public Student(int id, String name) {
		super();
		this.id = id;
		this.name = name;
	}
	public Student() {
		super();
	}
}

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public class Teacher {
	private int id;
	private String name;
    //getters and setters
    
	@Override
	public String toString() {
		return "Teacher [id=" + id + ", name=" + name + "]";
	}
	public Teacher(int id, String name) {
		super();
		this.id = id;
		this.name = name;
	}
	public Teacher() {
		super();
	}
}
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public class Classes {
	private int id;
	private String name;
	private Teacher teacher;
	private List<Student> students;
    //getters and setters
    
	@Override
	public String toString() {
		return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher + ", students=" + students + "]";
	}
	
	
	public Classes(int id, String name, Teacher teacher, List<Student> students) {
		super();
		this.id = id;
		this.name = name;
		this.teacher = teacher;
		this.students = students;
	}
	public Classes() {
		super();
	}
}

编写映射文件classesMapper.xml（关键）

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<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatis.test6.classesMapper">
	<!-- 
	方法一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
	SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND  c.c_id=1
	 -->
	<!-- 
		根据classId查询对应的班级信息,包括学生,老师
	 -->
	<select id="getClassesById" parameterType="int" resultMap="classesResultMap">
		select * from class c,teacher t,student s where c.teacher_id=t.t_id
		and
		s.class_id=c.c_id and c.c_id=#{id}
	</select>
	<resultMap type="Classes" id="classesResultMap">
		<id property="id" column="c_id" />
		<result property="name" column="c_name" />
		<association property="teacher" javaType="Teacher">
			<id property="id" column="t_id" />
			<result property="name" column="t_name" />
		</association>
		<!-- ofType指定students集合中的对象类型 -->
		<collection property="students" ofType="Student">
			<id property="id" column="s_id" />
			<result property="name" column="s_name" />
		</collection>
	</resultMap>
	<!-- 
		方法二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
			SELECT * FROM class WHERE c_id=1;
			SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值
			SELECT * FROM student WHERE class_id=1  //1是第一个查询得到的c_id字段的值
	 -->
	<!-- 
		查询所有的班级信息,包括学生,老师
	 -->
	<select id="getAllClasses" resultMap="allClassesResultMap">
		select * from class
	</select>
	<resultMap type="Classes" id="allClassesResultMap">
		<id property="id" column="c_id" />
		<result property="name" column="c_name" />
		<association property="teacher" column="teacher_id"
			select="getTeacherById">
		</association>
		<collection property="students" column="c_id"
			select="getStudentByClassId"></collection>
	</resultMap>
	<select id="getTeacherById" parameterType="int" resultType="Teacher">
		select t_name name from teacher where t_id=#{id}
	</select>
	<select id="getStudentByClassId" parameterType="int" resultType="Student">
		select s_id id,s_name name from student where class_id=#{id}
	</select>
</mapper>

这里提供了两种方法处理一对多的关联关系，和一对一中类似，分别是联表查询和分两次查询。关键点在于resultMap节点中的collection节点，注意在联表查询时，需要通过collection节点的ofType属性指定返回的集合的元素类型。