[FIXED] I am new to mybatis. I want to know how one to many relationship is mapped in mybatis

Issue

My project is all about crud operation using spring-mybatis. In which i am performing database operation on 1:M relationship table.Select query returns empty list. In Employee POJO class i have setters and getters for List skills = new ArrayList();

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>

Solution

the following should solve your problem(s):

please also set property javaType="List" in collection-tag

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

the ofType-property represents the generic Type of a Class / Interface; for example List<?>, you implement it as an ArrayList<Skill>, so javaType has to be List and ofType has to be skill

you declared the property columnPrefix in the collection-tag, but your select-statement has any columns prefixed with skill_. so you have to change/add something like s.skillid as skill_id, s.skillname as skill_name, s.empId as skill_empid

<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>

the declared columnPrefix, declared in collection-tag is 'added' automatically, to resolve the resultMap

for example, the select-statement declares a column named/labled skill_id

collection-tag tells myBatis to use a columnPrefix to resolve the declared resultMap

mybatis combines columnPrefix and the column-property of id-tags, resulttags, (whatever)

columnPrefix="skill_" and column="id" become

skill_id at runtime

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

Answered By - user4602302