Mybatis一对多集合映射始终具有一个默认实体 [英] Mybatis one-to-many collection mapping always have one default entity

问题描述

我想重写我们的服务以使用mybatis映射和连接来使我们的实体在数据库/ mybatis层上完整并完成。

I want to rewrite our services to use mybatis mapping and joins to have our entity full and completed on database/mybatis layer.

<resultMap id="ParentMap" type="org.example.mybatis.Parent">
    <id column="id" jdbcType="VARCHAR" property="id" />
    <id column="Name" jdbcType="VARCHAR" property="name" />
    <id column="SurName" jdbcType="VARCHAR" property="surName" />

    <collection property="childs" column="ChildId"
        javaType="ArrayList" ofType="org.example.mybatis.Child"
        resultMap="org.example.ChildMap" />    
</resultMap>

<resultMap id="ChildMap" type="org.example.mybatis.Parent">

    <id column="id" jdbcType="VARCHAR" property="id" />
    <id column="Name" jdbcType="VARCHAR" property="name" />
    <id column="SurName" jdbcType="VARCHAR" property="surName" />
    <id column="Age" jdbcType="INTEGER" property="age" />
</resultMap>

<sql id="Parent_Column_List">
    p.Id, p.Name, p.SurName,
</sql>  

<sql id="Child_Column_List">
    c.Id, c.ParentId c.Name, c.SurName, c.Age
</sql>  

<select id="getParent" parameterType="java.lang.String" resultMap="ParentMap" >
    select 
    <include refid="Parent_Column_List"/>

    <include refid="Child_Column_List" />
    from Parent p

    left outer join Child c on p.Id = c.ParentId
    where p.id = #{id,jdbcType=VARCHAR}

问题是下一个：如果父母没有孩子，一些带有null或default字段的默认实体将被添加到列表中。
我明白这是外连接的本质，但mybatis不是很聪明地明白这是假的吗？

Problem is next: if parent doesn't has childs, some default entity with null or default fields will be added to list. I understand that this is nature of outer join, but is mybatis not very clever to understand that this is fake?

是否有一些解决方法？我不能使用内连接，因为父实体必须在结果中。

Is there some workaround about this? I cannot use inner join since parent entity is required to be in result.

推荐答案

你必须把属性 notNullColumn 在你的收藏中。所以你的resultMap将是：

You have to put the attribute notNullColumn in your collection. So your resultMap will be:

<resultMap id="ParentMap" type="org.example.mybatis.Parent">
    <id column="id" jdbcType="VARCHAR" property="id" />
    <id column="Name" jdbcType="VARCHAR" property="name" />
    <id column="SurName" jdbcType="VARCHAR" property="surName" />

    <collection property="childs" column="ChildId" notNullColumn="id"
        javaType="ArrayList" ofType="org.example.mybatis.Child"
        resultMap="org.example.ChildMap" />    
</resultMap>

请注意，您可能还会遇到两个 id ，所以你可能必须在你的选择

Note that you will also maybe have issues with the two id, so you will maybe have to have a c.id as ChildId in your select

这篇关于Mybatis一对多集合映射始终具有一个默认实体的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！