Objects instantiated when all fields in a ResultMap are null

[jpeffer]

Empty objects are instantiated when all fields in a ResultMap are null and an ID tag is present, regardless of whether that defined ID is mapped to a property.

Consider the following ResultMap definition:

<resultMap id="employeesInEstablishment" type="Person">
    <id column="establishment_id"/>
    <result property="firstName"  column="owner_first_name" />
    <result property="lastName"  column="owner_last_name" />
</resultMap>

This result map has a 1-many relationship with an Establishment, which has an establishment ID. The select statement and result maps are much more complex than this, but I am trying to simplify the issue down to what I believe to be the root cause.

When retrieving establishment results, regardless of dealing with an association or collection, I end up with empty objects within my Establishment, even if, in the example above, first and last name columns are null.

When the ID tag is removed, this behavior does not occur, but after profiling the application, the number of invocations on resultSet.getString literally doubles, from ~11k to ~26k.

It would make sense for MyBatis to ignore IDs that are not mapped to a property within the associated resultMap type's object, when deciding to instantiate the object or not. This would allow for mapping to be optimized while avoiding instantiation of empty objects, which later causes serialization issues to json.

[yankee42]

I have encountered the same behaviour for collections. This has changed between version 1.2.8 and 1.3.0. I created an example repository with a full demonstration. It has three unit tests which all work fine on version 1.2.8 (first commit), but one of the test fails in version 1.3.0 (second commit)

In my opinion the documentation is not clear about whether the <id> tag should be there and thus whether this is a bug or desired behaviour. What is it? (So that I know whether to wait for the next version or upgrade now and change my mappings).

For reference (since the linked repository may not be there forever) here the important parts:
In this example we have a model for TODO-Lists like this:

Table lists: id
Table items: listId (references table lists), order, description (primary key: listId+order)

So there are a number of lists and each list as items which have order and description and a listId+order is unique.

Here are queries and mappings for HSQL 2.3.3:

<mapper namespace="emptycollection.Dao">
    <select id="selectWithEmptyList" resultMap="...">
        select 1 id, null "order", null description from (VALUES ('dummy')) dummy(dummy)
    </select>

    <select id="selectWithNonEmptyList" resultMap="...">
        select 1 id, 1 "order", 'a description' description from (VALUES ('dummy')) dummy(dummy)
        union all select 1 id, 2 "order", 'a 2nd description' description from (VALUES ('dummy')) dummy(dummy)
        union all select 2 id, 1 "order", 'a description' description from (VALUES ('dummy')) dummy(dummy)
    </select>

    <resultMap id="selectList" type="emptycollection.TodoLists">
        <id column="id" property="id" />
        <collection property="todoItems" ofType="emptycollection.TodoItem" javaType="list">
            <id column="id" />
            <id column="order" property="order" />
            <result column="description" property="description"/>
        </collection>
    </resultMap>

    <resultMap id="selectList_noCollectionId" type="emptycollection.TodoLists">
        <id column="id" property="id" />
        <collection property="todoItems" ofType="emptycollection.TodoItem" javaType="list">
            <id column="order" property="order" />
            <result column="description" property="description"/>
        </collection>
    </resultMap>

</mapper>

Query	Mapping	MyBatis Version	Result
selectWithEmptyList	selectList	1.2.8	OK: [List(1)=[]]
selectWithEmptyList	selectList_noCollectionId	1.2.8	OK: [List(1)=[]]
selectWithNonEmptyList	selectList	1.2.8	OK*
selectWithNonEmptyList	selectList_noCollectionId	1.2.8	OK*
selectWithEmptyList	selectList	1.3.0	FAIL: [List(1)=[null(null)]]
selectWithEmptyList	selectList_noCollectionId	1.3.0	OK: [List(1)=[]]
selectWithNonEmptyList	selectList	1.3.0	OK*
selectWithNonEmptyList	selectList_noCollectionId	1.3.0	OK*

* Expected was: [a description(1), a 2nd description(2)], List(2)=[a description(1)]]

[emacarron]

Hi both,

@yankee42 I have checked your test case and I was not able to make it fail. I am afraid you forgot to push the two pojos so maybe I wasn't able to reproduce the case in the same exact way that you did in your repo.

I have uploaded a test case to current master so we can all work on it. Please the the commit above.

It would be great if you could fork the project, change the test so it fails and let me know. This issue should be easy to fix, it is just a matter of understanding the problem.

[yankee42]

Hi @emacarron,

that was pretty stupid from my side to not add the pojos. I did so know if you want to have a look again.

For your tests the only thing that you are missing is

final List<TodoItem> todoItems = actual.get(0).getTodoItems();
Assert.assertEquals("expect " + todoItems + " to be empty", 0, todoItems.size());

in the testWithEmptyList() method. See #541 for PR.

[emacarron]

Hi @yankee42

I have been able to reproduce the problem without problems. Hope now it is fixed. Thank you!