Mybatis 3.0.5 nested collection mapping Example

Question

I am investigating the mapping facilities of MyBatis 3.0.5. The database is H2 (1.3.160) in running embedded mode. With the help of the user manual, I got the straightforward parts working. But I am having a hard time mapping a Set that uses a HashMap as the backing storage.

Here's the Java code for the custom collection that has the custom set as field (simplified for brevity)

public class CustomCollection 
{
    @JsonProperty
    private CustomSet<CustomItem> customItems;

    public CustomCollection()
    {
        customItems = new CustomSet<CustomItem>();
    }

    // other stuff  
}

Here's the CustomSet code (again, simplified)

public class CustomSet<E extends CustomItemInterface> extends AbstractSet<E>
{
    private ConcurrentHashMap<String, E> items;

    public CustomSet()
    {
        items = new ConcurrentHashMap<String, E>();
    }

    // other stuff  
}

Here's the mapping interface:

public interface CustomCollectionMapper 
{
    CustomCollection select(@Param("somename") String s1, @Param("someothername") String s2);
}

This is the code that does the call into the Mybatis framework:

SqlSessionFactory sqlSessionFactory = (SqlSessionFactory) servletContext.getAttribute("SqlSessionFactory");
SqlSession session = sqlSessionFactory.openSession();
CustomCollection coll = null;
try 
{ 
    CustomCollectionMapper mapper = session.getMapper(CustomCollectionMapper.class);
    coll = mapper.select(param1, param2);
} 
finally 
{ 
    session.close(); 
} 

Here's what I could come up with the mapping XML so far:

<select id="select" resultMap="CustomCollectionMapping">
-- What goes here???
</select>

<resultMap type="com.example.CustomCollection" id="CustomCollectionMapping">
  <association property="customItems" javaType="com.example.customSet">
    <collection property="items" javaType="HashMap" ofType="com.example.CustomItem" select="selectCustomItems">
    </collection>
  </association>
</resultMap>

<select id="selectCustomItems" parameterType="map" resultType="com.example.CustomItem">  
  -- SQL query to return multiple CustomItem rows
</select>  

Through various iterations, I got "too many results" error, some other error or nothing (null is returned from the mapper call) but never the result I need. The SQL code works fine by itself and if I ask for a List with a straightforward select statement, I get the rows and the ArrayList back. The problem I am having is having the nested collection objects populated properly.

I have read the manual many times, searched for examples but I could not come up with the correct mapping XML for this purpose. I would appreciate if anyone can help me or point me to a source that can help.

Thanks in advance for all the help.

Answer 1

This is my working example:

<resultMap id="categoryPreferenceValueMap" type="SyncCategoryPreferenceValueModel">
         <id property="preferenceTypeId" column="preference_type_id" />
         <result property="title" column="title"/>
         <result property="index" column="type_index"/>
          <result property="updatedAt" column="category_updated_at" />
          <collection property="preferences" column="p_preference_id" ofType="SyncPreferenceModel" >
            <id property="preferenceId" column="p_preference_id" />
            <result property="title" column="p_preference_title" />
            <result property="index" column="preference_index" />
            <result property="updatedAt" column="preference_updated_at" />
            <collection property="preferenceValues" column="p_v_preference_value_id"  ofType="SyncPreferenceValueModel"  >
                <id property="preferenceValueId" column="p_v_preference_value_id" />
                <result property="preferenceValue" column="p_v_preference_value" />
                <result property="updatedAt" column="preference_value_updated_at" />
            </collection>  
         </collection>
    </resultMap>

This is my query

<select id="getPrefersenceWithValues" resultMap="categoryPreferenceValueMap">
    SELECT  
        PT.preference_type_id, PT.title, PT.type_index,PT.updated_at as  category_updated_at,
        P.preference_id as p_preference_id , P.title as p_preference_title  ,P.index as preference_index,

        P.updated_at as preference_updated_at,

        PV.preference_value_id as p_v_preference_value_id ,PV.preference_value as p_v_preference_value  

    FROM preference_types PT
    INNER JOIN preferences P ON PT.preference_type_id=P.preference_type_id 
        INNER JOIN preference_values PV ON P.preference_id=PV.preference_id 

    ORDER BY type_index
</select>

And output is:

 [
    {
      "preferenceTypeId": "1",
      "title": "abc BASICS",
      "index": "1",
      "updatedAt": 1,
      "preferences": [
        {
          "preferenceId": "1",
          "title": "xyz xyz",
          "preferenceTypeId": null,
          "gender": null,
          "index": 1,
          "updatedAt": 1,
          "preferenceValues": [
            {
              "preferenceId": null,
              "preferenceValueId": "2",
              "preferenceValue": "30-60",
              "gender": null,
              "updatedAt": 0
            },
            {
              "preferenceId": null,
              "preferenceValueId": "1",
              "preferenceValue": "0-30",
              "gender": null,
              "updatedAt": 0
            }
          ]
        }
      ]
    }
  ]