[IBATIS-375] Provide a way for not creating items from nested ResultMaps when the items contain only null values

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.2.0
• Fix Version/s: 2.3.4
• Component/s: Core
• Assignee: Unassigned
• Reporter: Stefan Langer
• Created: 01/Dec/06 08:45
• Updated: 15/Aug/09 13:33
• Resolved: 18/Jul/08 06:37

Description

Taken the following resultmaps (uneeded parts are left out) for a simple domainmodel where notebook contains 0 or more pages.

<resultMap class="notebook" groupBy="id" id="notebookResult">
<result column="NB_ID" jdbcType="VARCHAR" property="id"/>
<result javaType="java.util.List" property="pages" resultMap="Note.pageResult"/>
</resultMap>

<resultMap class="page" groupBy="id" id="pageResult">
<result column="P_ID" jdbcType="VARCHAR" property="id"/>
</resultMap>

and the following select statement:
<select id="selectNotebook" parameterClass="java.lang.String" resultMap="notebookResult">
SELECT nb.id as nb_id, p.id as p_id,
FROM notebook nb LEFT JOIN page p ON nb.id = p.notebook_id
WHERE nb.id = #id#
</select>

Do to the left join this query will always result in at least one page being created even though the pages are actually empty.
It would be nice if the mapping framework could somehow not create these empty objects.
The following suggestions come to mind:
Either do not create objects where all parameters are null (this indicates the left or right join scenario)
or provide a way to tag a property in the resultMap which may not be null in order for the object to be created.
Example:
<resultMap class="page" groupBy="id" id="pageResult">
<result column="P_ID" jdbcType="VARCHAR" property="id" doNotCreateOnNull="true"/>
</resultMap>
This would greatly ease the use of complex mappings where one wants to prevent the dreaded N+1 Select Problem.

Issue Links

• relates to: IBATISNET-206 groupBy incorrectly handles left outer join results where right part is empty