MyBatis Polymorphic Collections with Abstract Base Class

Problem

Straight from the MyBatis manual:

<!-- Very Complex Statement --> 
<select id="selectBlogDetails" parameterType="int" resultMap="detailedBlogResultMap"> 
  select 
    B.id as blog_id,
    B.title as blog_title, 
    B.author_id as blog_author_id, 
    A.id as author_id, 
    A.username as author_username, 
    A.password as author_password, 
    A.email as author_email, 
    A.bio as author_bio, 
    A.favourite_section as author_favourite_section, 
    P.id as post_id, 
    P.blog_id as post_blog_id, 
    P.author_id as post_author_id, 
    P.created_on as post_created_on, 
    P.section as post_section, 
    P.subject as post_subject, 
    P.draft as draft, 
    P.body as post_body, 
    C.id as comment_id, 
    C.post_id as comment_post_id, 
    C.name as comment_name, 
    C.comment as comment_text, 
    T.id as tag_id, 
    T.name as tag_name 
  from Blog B 
    left outer join Author A on B.author_id = A.id 
    left outer join Post P on B.id = P.blog_id 
    left outer join Comment C on P.id = C.post_id 
    left outer join Post_Tag PT on PT.post_id = P.id 
    left outer join Tag T on PT.tag_id = T.id 
  where B.id = #{id} 
</select>

<!-- Very Complex Result Map --> 
<resultmap id="detailedBlogResultMap" type="Blog">
<constructor> 
    <idarg column="blog_id" javaType="int"/>  
  </constructor><result property="title" column="blog_title"/> 
  <association property="author" column="blog_author_id" javaType=" Author"> 
    <id property="id" column="author_id"/> 
    <result property="username" column="author_username"/> 
    <result property="password" column="author_password"/> 
    <result property="email" column="author_email"/> 
    <result property="bio" column="author_bio"/> 
    <result property="favouriteSection" column="author_favourite_section"/> 
  </association> 
  <collection property="posts" ofType="Post"> 
    <id property="id" column="post_id"/> 
    <result property="subject" column="post_subject"/> 
    <association property="author" column="post_author_id" javaType="Author"/> 
    <collection property="comments" column="post_id" ofType=" Comment"> 
      <id property="id" column="comment_id"/> 
    </collection> 
    <collection property="tags" column="post_id" ofType=" Tag" > 
      <id property="id" column="tag_id"/> 
    </collection>
    <discriminator javaType="int" column="draft"> 
      <case value="1" resultType="DraftPost"/> 
    </discriminator> 
  </collection> 
</resultMap>

Unfortunately, if Post is abstract, using such a result map would cause exceptions to be thrown during object construction when a left join to the Post table yields missing records in the resultset.

The problem appears to be rooted in the usage of the null object pattern during construction from nested results, particularly an invocation of newInstance() on the type specified in the ofType attribute to produce a temporary placeholder. Of course, if that type happens to be abstract, you'll be seeing a strack trace sooner rather than later.

Solution

Given that the placeholder object is discarded when a more suitable match is available, what better object than an actual Object to fill this gap:

  <collection property="posts" ofType="object">

Problem solved.