1. Requirements
For example, we have two objects, one question and answer, one comment, which is similar to what Baidu knows; when we query a question and answer, you can see the comments below;
I'm looking for J_When questionandanswer this table, you need to query j_comment this table and put j_comment The data in this table is placed in the entity questionAndAnswer; the steps are as follows:
2. Solutions
Dead work:
1. j_Creation of comment table;
2. Entity class creation for comment
3. In j_Queionandanswer (alias qa) A left join is added to this table:
left join j_comment jc on jc.titleid = qa.id
4. In select, add what you want in j_The field queried in the comment table; jc is j_Alias for comment;
5. In the QuestionAndAnswer entity, add the List collection of Comment mappings: 
6. Note: IDS in jc tables conflict with IDS in qa tables.jc.id The alias is cid;
<!--Paging Query--> <select id="selectByProperties" parameterType="com.jxdx.questionAndAnswer.model.QuestionAndAnswer" resultMap="BaseResultMap"> select <include refid="Base_Column_List_Alias"/>,jc.id AS cid, jc.titlename, jc.titleid, jc.username, jc.userid, jc.comment, jc.create_date from jxdx_questionandanswer qa <if test="studentid != null"> left join sys_t_user u on u.id = qa.studentid </if> <if test="teacherid != null"> left join sys_t_user u on u.id = qa.teacherid </if> left join jxdx_comment jc on jc.titleid = qa.id where qa.deleted=0 <if test="name!=null"> AND qa.name like '%$%' </if> <if test="studentid!=null"> AND qa.studentid=# </if> <if test="studentname!=null"> AND qa.studentname=# </if> <if test="createDate != null"> AND qa.create_date <![CDATA[>=]]> # </if> <if test="updateDate != null"> AND qa.update_date <![CDATA[>=]]> # </if> <!-- <if test="updateDate != null"> AND qa.update_date <![CDATA[<=]]> date_add(#, INTERVAL 1 DAY) </if>--> <if test="teacherid != null"> AND qa.teacherid = # </if> <if test="teachername!=null"> AND qa.teachername=# </if> <if test="studentclassid!=null"> AND qa.studentclassid=# </if> <if test="studentclassname!=null"> AND qa.studentclassname=# </if> <if test="question!=null"> AND qa.question=# </if> <if test="answer!=null"> AND qa.answer=# </if> <if test="status!=null"> AND qa.status=# </if> <if test="remark!=null"> AND qa.remark=# </if> <if test="deleted!=null"> AND qa.deleted=# </if> <if test="createBy != null"> AND qa.createBy = # </if> <if test="updateBy != null"> AND qa.updateBy = # </if> order by qa.create_date desc </select>
Note:
Here <include refid="Base_Column_List_Alias "/>
Is a collection of fields that are queried and can be replaced with specific field names;` AND qa.create_date
<collection property="commentList" ofType="com.jxdx.questionAndAnswer.model.Comment" notNullColumn="id"> <id column="cid" property="id" jdbcType="BIGINT"/> <result column="titlename" property="titleName" jdbcType="VARCHAR" /> <result column="titleid" property="titleId" jdbcType="VARCHAR"/> <result column="username" property="userName" jdbcType="BIGINT" /> <result column="userid" property="userId" jdbcType="VARCHAR" /> <result column="comment" property="comment" jdbcType="VARCHAR"/> <result column="remark" property="remark" jdbcType="VARCHAR"/> <result column="create_date" property="createDate" jdbcType="VARCHAR"/> </collection>
complete