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<resultMap id="SandboxMap" type="Sandbox">
    <id property="firstName" column="FIRSTNAME"/>
    <result property="age" column="AGE"/>
    <collection property="split" javaType="list" ofType="Split" column="AGE" select="selectSplit">
        <id property="sandboxId" column="SANDBOX_ID"/>
    </collection>
</resultMap>
<select id="selectSandbox" resultMap="SandboxMap">
    SELECT * FROM SANDBOX
    WHERE AGE=#{age}
</select>
<select id="selectSplit" parameterType="int" resultType="com.scibor.Split">
    SELECT * FROM SPLIT
    WHERE Split.SANDBOX_ID=#{age}
</select>
And I have in my Database some entries such that there is a Sandbox with Age 14 and there are a few Splits whose SANDBOX_ID is 14. There are no imposed constraints on any of the columns because it was not specified in the documentation that I had to do so, i.e. these are all simple data types, NUMBER and VARCHAR no notion of PRIMARY KEY, NOT NULL etc.
Within my code I then invoke:
Sandbox sandbox = session.selectOne("Sandbox.selectSandbox", 14);
This returns a Sandbox object whose Name and Age fields are properly set, however, the array associated with splits it's just an array of nulls. If I was to guarantee uniqueness and change collection to association, it would be the same thing, split would just be null. 
I've been staring at this for some time and I have tried all sorts of values for column in collection tag, as well as various variants of the selectSplit entry, but none of them work, they always return a null value for split. More annoyingly, no exceptions are thrown so it seems that all of the SQL queries being generated are in fact valid (most likely the last #{age} is itself null).
Can someone please explain how I can retrieve an instance of Sandbox with a List<Split> populated using MyBatis? The documentation is helpful but I have followed the examples, I think exactly, yet I am still not getting the expected result.
It turns out that the difficulty was in this line:
<select id="selectSplit" parameterType="int" resultType="com.scibor.Split">
One of the things I had taken for granted was that the property names in Split do not match up with the names of the columns for the associated table. So instead of using a resultType, I had to create the following: 
<resultMap id="SplitMap" type="Split">
    <id column="SANDBOX_ID" property="sandboxId"/>
</resultMap>
This resolved the issue.
        
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