Mybatis map collection to HashMap

Question

I'm rather new to MyBatis. There's theFoo which could have many Bars, and the Bar has fooId as FK. I want to query for all Foos with its associated Bars.

Here are my domain objects:

public class Foo {
    private int id;
    private String name;
    // key: Bar.id, value: Bar instance
    private Map<Integer, Bar> bars;

    // getter, setter etc
}

public class Bar {
    private int id;
    private int fooId;
    private String model;

    // getter, setter etc
}

The schema:

Foo
id	int	PK
name	varchar

Bar
id	int	PK
fooId	varchar	FK
model	varchar

The result maps:

<resultMap id="selectBar" type="Bar">
    <id property="id" column="id"/>
    <result property="fooId" column="fooid"/>
    <result property="model" column="model"/>
</resultMap>

<resultMap id="fooWithBars" type="Foo">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="bars" ofType="Bar" resultMap="Bar.selectBar" javaType="java.util.Map"/>
</resultMap>

The query:

<select id="selectFoos" resultMap="fooWithBars">
    select F.name, F.id, B.id, B.fooId, B.model
    from Foo P join Bar B on F.id = B.fooId
</select>

My question is how to map the collection to Map<Integer, Bar>? without using ResultHandler The resultMap above apparently doesn't work. I also tried:

nested resultMap which end up with collection of HashMaps instead of a collection in Hashmap, and there's no way of specifying the key to be the value of Bar.id
constructor injection which failed since Mybatis does not support use collection as an argumenthttps://github.com/mybatis/mybatis-3/issues/101

Thanks!

You may try something with @MapKey ... It looks like it doesn't work directly inside of XML mapper, but nevertheless it could lead you on the right track. Check https://stackoverflow.com/questions/11913013/return-hashmap-in-mybatis-and-use-it-as-modelattribute-in-spring-mvc and https://mybatis.org/mybatis-3/apidocs/org/apache/ibatis/annotations/MapKey.html — BrunoT, Apr 22 '22 at 14:36

Clement Cherlin · Answer 1 · 2023-02-06T22:23:30.510

While MyBatis doesn't natively support maps, it does support sets, and it's relatively straightforward to create an adapter that makes a map's entrySet writeable, allowing MyBatis to add elements to it.

AddEntrySet.java

import java.util.*;

public class AddEntrySet<K,V> extends AbstractSet<Map.Entry<K, V>> {
    private final Map<K, V> map;

    public AddEntrySet(Map<K, V> map) {
        this.map = map;
    }

    @Override
    public Iterator<Map.Entry<K, V>> iterator() {
        return map.entrySet().iterator();
    }

    @Override
    public int size() {
        return map.size();
    }

    @Override
    public boolean add(Map.Entry<K, V> kvEntry) {
        V value = kvEntry.getValue();
        return !Objects.equals(value, map.put(kvEntry.getKey(), value));
    }
}

Foo.java

    // For MyBatis to add through
    private Set<Map.Entry<Integer, Bar>> getBarEntries() {
        return new AddEntrySet<>(bars);
    }

Result maps

  <resultMap id="selectBar" type="Bar">
    <id property="id" column="id"/>
    <result property="fooId" column="fooid"/>
    <result property="model" column="model"/>
  </resultMap>

  <resultMap id="barEntry" type="java.util.AbstractMap$SimpleImmutableEntry">
    <constructor>
      <idArg javaType="java.lang.Object" column="id"/>
      <arg javaType="java.lang.Object" resultMap="selectBar"/>
    </constructor>
  </resultMap>

  <resultMap id="fooWithBars" type="Foo">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <collection property="barEntries" columnPrefix="bar_" ofType="java.util.AbstractMap$SimpleImmutableEntry" resultMap="barEntry"/>
  </resultMap>

Query

select F.name, F.id, B.id AS bar_id, B.fooId AS bar_fooId, B.model AS bar_model
from Foo F left join Bar B on F.id = B.fooId

Mybatis map collection to HashMap

1 Answers1

AddEntrySet.java

Foo.java

Result maps

Query