我是mybatis的新手。我想知道一对多关系在mybatis中是如何映射的

Question

我的项目都是关于使用spring-mybatis的crud操作。其中我正在对1:M relationship执行数据库操作，table.Select查询返回空列表。在Employee POJO类中，我有用于List skills = new ArrayList()的setter和getter；

Mapper.xml

<resultMap type="employee" id="result">
    <id property="employeeId" column="empId" />
    <result property="firstName" column="firstName" />
    <result property="lastName" column="lastName" />
    <result property="age" column="age" />
    <result property="gender" column="gender" />
    <result property="salary" column="salary" />
    <result property="department" column="department" />
    <result property="state" column="state" />
    <result property="city" column="city" />
    <result property="skillSet" column="skillSet" />
    <result property="address" column="address" />
    <result property="email" column="email" />
    <collection property="skills" ofType="skill" resultMap="skillResult" columnPrefix="skill_"></collection>
</resultMap>

<resultMap type="skill" id="skillResult">
    <id property="skillId" column="skillId"/>
    <result property="skillname" column="skillname"/>
    <result property="empId" column="empId"/>
</resultMap>

<select id="getAllEmployees" resultType="employee" resultMap="result">
    Select e.empid,e.firstname,e.lastname,e.age,e.salary,e.department,e.state,e.city,e.address,e.gender,e.email,s.skillname,s.empId
    from Employee40 e right outer join Skill s on e.empid = s.empid
</select>

Answer 1

以下方法可以解决您的问题：

请同时在collection-tag中设置属性javaType="List"

<collection
    property="skills"
    javaType="List"
    ofType="skill"
    resultMap="skillResult"
    columnPrefix="skill_"/>

ofType-property表示Class /Interface的泛型代码；例如List<?>，您将其实现为ArrayList<Skill>，因此javaType必须为List，ofType必须为<代码>D12

您在collection-tag中声明了属性columnPrefix，但是您的select语句中有任何以skill_为前缀的列。因此，您必须更改/添加s.skillid as skill_id, s.skillname as skill_name, s.empId as skill_empid之类的内容

<select id="getAllEmployees" resultType="employee" resultMap="result">
    Select
        e.empid,
        e.firstname,
        e.lastname,
        e.age,
        e.salary,
        e.department,
        e.state,
        e.city,
        e.address,
        e.gender,
        e.email,
        s.skillid   as skill_id,
        s.skillname as skill_name,
        s.empId     as skill_empid
    from
        Employee40 e
    right outer join
        Skill s
    on e.empid = s.empid
</select>

在collection-tag中声明的声明的columnPrefix是自动添加的，以解析resultMap

例如，select-语句声明名为/标记为skill_id的列

collection-tag告诉myBatis使用columnPrefix来解析声明的resultMap

mybatis结合了columnPrefix和column-property of id-tags，resulttags，(诸如此类)

columnPrefix="skill_"和column="id"成为

运行时的skill_id

<resultMap type="skill" id="skillResult">
    <id
        property="skillId"
        column="id"/>
    <result
        property="skillname"
        column="name"/>
    <result
        property="empId"
        column="empId"/>
</resultMap>