MyBatis | Mapping Query Results to Arbitrary Java Objects | Collection Mapping

Tags:

Basic usage

DB tables

foo_table

id
1
2
3

bar_table

key1 foo_id
1 1
2 1
3 1
4 2
5 2
6 3

Source code

Foo.java

package sample.mybatis;

import java.util.List;

public class Foo {
    private int id;
    private List<Bar> barList;

    @Override
    public String toString() {
        return "Foo [id=" + id + ", barList=" + barList + "]";
    }
}

Bar.java

package sample.mybatis;

public class Bar {
    private int id;

    @Override
    public String toString() {
        return "Bar [id=" + id + "]";
    }
}

sample_mapper.xml

<?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="sample.mybatis">

  <resultMap type="sample.mybatis.Foo" id="fooResultMap">
    <id property="id" column="id" />
    <!-- Use the collection tag. -->
    <collection property="barList" ofType="sample.mybatis.Bar">
      <id property="id" column="bar_id" />
    </collection>
  </resultMap>

  <select id="selectFoo" resultMap="fooResultMap">
    select foo.id
          ,bar.id bar_id
      from foo_table foo
          ,bar_table bar
     where bar.foo_id = foo.id
  </select>
</mapper>

Execution result of selectFoo

[DEBUG] s.m.selectFoo   - ==>  Preparing: select foo.id ,bar.id bar_id from foo_table foo ,bar_table bar where bar.foo_id = foo.id 
[DEBUG] s.m.selectFoo   - ==> Parameters: 
[DEBUG] s.m.selectFoo   - <==      Total: 6
Foo [id=1, barList=[Bar [id=1], Bar [id=2], Bar [id=3]]]
Foo [id=2, barList=[Bar [id=4], Bar [id=5]]]
Foo [id=3, barList=[Bar [id=6]]]

Explanation

  • Use the <collection> tag to map a collection.
  • Specify the type of each element with the ofType attribute. Aliases can also be used.
  • Otherwise, configure it in the same way as the <association> tag.