Mybatis Association as one-one instead of one-many - mybatis

Question

Edited for clarity 02/07/17
I'm using Mybatis 3.3 at work, and I've run into a roadblock. I'm pretty sure it's a problem with my resultMapper, but I'm having some difficulty finding relevant tutorials/info.
I have an existing Java model, Mybatis mappers, and tables; and I'm trying to write a new module that reuses as much as possible. My existing model looks like this:
class Document {
Header header;
List<Detail> details;
}
I want to reuse the model with a different Mybatis mapper to produce a 1-1 relationship between Details and Headers (i.e. details.size() is always 1).
I can currently only get 1 Document. It pulls the first header in the table, and it attaches every detail of every document to it. Here are my result maps and the query I'm working on. The query returns the correct results, but Mybatis wraps them incorrectly.
<resultMap id="header" type="Header">
<result property="id" column="ID" />
<result property="title" column="TITLE" />
</resultMap>
<resultMap id="detail" type="Detail">
<result property="id" column="ID" />
<result property="title" column="INFO" />
</resultMap>
<resultMap id="document" type="Document">
<association property="header" resultMap="header" />
<association property="details" resultMap="detail" />
</resultMap>
SELECT
HEADER.ID,
DETAIL.ID
FROM HEADER
JOIN DETAIL ON HEADER.ID = DETAIL.HEADER_ID

Answer 1

In document resultMap use collection instead of association for property details.
EDIT 02-08-17:
According to your edit that makes things clearer: a slight model change would match more naturally with your need:
class Document {
Header header;
Detail detail;
}
But you want to keep the List<Detail> of 1 element.
I thought about that. It just requires adding a "virtual property" => getter and setter to wrap the list.
public void setSingleDetail(Detail detail) {
this.details = Arrays.asList(detail);
}
public Detail getSingleDetail() {
return null == this.details ? null : this.details.iterator().next();
}
But that is not enough, because it just overwrites the detail list.
If have a look to the method org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleRowValuesForNestedResultMap. You will figure out that Mybatis does not allow what you want, it will always "group by" container entity (if you do not specify id property, it will uses hash). That is also why the getter is required: to compare with values from new row.
What you want to do requires a resultMap Detail with association to Header (and of course a class with matching structure), then you get a result / Detail.

Answer 2

To map row set to a tree of objects crucial thing is identity of the objects. Consider this example:
header_id | detail_id
---------------------
1 | 1
1 | 2
There are two ways to parse this to object tree with Document as a root entity. The first way is to have one entity with header and a collection of two Details. Another way is to have two Documents each having one Detail. The difference here is in identity of the Document objects. You need to tell mybatis what column is the identifier for Document entity.
For your purpose this should be id of the Detail.
You would need to change the query and mapping to make this possible.
The main purpose is to specify detail id as a document id in this mapping:
<resultMap id="document" type="Document">
<id column="detail_id"/>
<association property="header" resultMap="header" />
<association property="details" resultMap="detail"/>
</resultMap> modify the query
You would need to change the query so that detail id column has unique name, like this
SELECT
HEADER.ID,
DETAIL.ID DETAIL_ID
FROM HEADER
JOIN DETAIL ON HEADER.ID = DETAIL.HEADER_ID
But this will break mapping for other columns in Detail association.
In order to use resultMap for Detail you would need to add the same prefix for all columns from detail table and specify that in mapping:
<resultMap id="document" type="Document">
<id column="detail_id"/>
<association property="header" resultMap="header" />
<association property="details" resultMap="detail" prefix="DETAIL_"/>
</resultMap> modify the query
Note that prefix should be upper case due to mybatis bug.