Try something like this:
<dataConfig>
<!-- dataSource is just an example. Included just for completeness. -->
<dataSource batchSize="500" type="JdbcDataSource" driver="com.mysql.jdbc.Driver" url="jdbc:mysql://localhost/my-database" user="root" password="somethinglong1283"/>
<document>
<entity name="comment" pk="id" query="SELECT * FROM comment">
<field column="blogpost_id" name="blogpost_id"/>
<field column="comment_text" name="comment_text" />
<entity name="comment_tags" pk="comment_id" query="SELECT * FROM comment_tags WHERE comment_id='${comment.id}'">
<field column="tag" name="tag" />
</entity>
</entity>
</document>