MyBatis中标签的多种应用场景与用法解析

在MyBatis中，<collection>标签用于处理一对多或多对多的关系映射，即将数据库中的多条记录映射为Java对象中的一个集合属性。<collection>标签可以用于映射集合类型包括List、Set、Array等。以下是<collection>标签的几种常见用法：

基本用法：

通过<collection>标签，指定Java对象的集合属性，以及对应的数据库表和字段。
常用于映射一个对象中包含的多个子对象。


<resultMap id="personResultMap" type="Person">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <collection property="orders" ofType="Order">
        <id property="id" column="order_id" />
        <result property="amount" column="amount" />
    </collection>
</resultMap>
<resultMap id="personResultMap" type="Person">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <collection property="orders" ofType="Order">
        <id property="id" column="order_id" />
        <result property="amount" column="amount" />
    </collection>
</resultMap>
<resultMap id="personResultMap" type="Person">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <collection property="orders" ofType="Order">
        <id property="id" column="order_id" />
        <result property="amount" column="amount" />
    </collection>
</resultMap>

上述例子中，一个Person对象包含多个Order对象。

嵌套查询：

使用select属性指定一个子查询来获取集合中的元素。
适用于需要通过其他查询获取关联数据的场景。


<resultMap id="personResultMap" type="Person">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <collection property="orders" ofType="Order" select="selectOrdersByPersonId" column="id" />
</resultMap>
<select id="selectOrdersByPersonId" resultType="Order">
    SELECT id, amount FROM orders WHERE person_id = #{personId}
</select>
<resultMap id="personResultMap" type="Person">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <collection property="orders" ofType="Order" select="selectOrdersByPersonId" column="id" />
</resultMap>

<select id="selectOrdersByPersonId" resultType="Order">
    SELECT id, amount FROM orders WHERE person_id = #{personId}
</select>
<resultMap id="personResultMap" type="Person">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <collection property="orders" ofType="Order" select="selectOrdersByPersonId" column="id" />
</resultMap>

<select id="selectOrdersByPersonId" resultType="Order">
    SELECT id, amount FROM orders WHERE person_id = #{personId}
</select>

在这个例子中，selectOrdersByPersonId是一个子查询，用于根据person_id检索订单。

嵌套结果：

当关联数据可以在同一个查询中获取时，使用嵌套结果来避免额外的查询。
通过<resultMap>中的<collection>和子对象的<resultMap>配置。


<select id="selectPersonWithOrders" resultMap="personWithOrdersResultMap">
    SELECT p.id AS person_id, p.name AS person_name, o.id AS order_id, o.amount AS order_amount
    FROM persons p
    LEFT JOIN orders o ON p.id = o.person_id
</select>
<resultMap id="personWithOrdersResultMap" type="Person">
    <id property="id" column="person_id" />
    <result property="name" column="person_name" />
    <collection property="orders" ofType="Order">
        <id property="id" column="order_id" />
        <result property="amount" column="order_amount" />
    </collection>
</resultMap>
<select id="selectPersonWithOrders" resultMap="personWithOrdersResultMap">
    SELECT p.id AS person_id, p.name AS person_name, o.id AS order_id, o.amount AS order_amount
    FROM persons p
    LEFT JOIN orders o ON p.id = o.person_id
</select>

<resultMap id="personWithOrdersResultMap" type="Person">
    <id property="id" column="person_id" />
    <result property="name" column="person_name" />
    <collection property="orders" ofType="Order">
        <id property="id" column="order_id" />
        <result property="amount" column="order_amount" />
    </collection>
</resultMap>
<select id="selectPersonWithOrders" resultMap="personWithOrdersResultMap">
    SELECT p.id AS person_id, p.name AS person_name, o.id AS order_id, o.amount AS order_amount
    FROM persons p
    LEFT JOIN orders o ON p.id = o.person_id
</select>

<resultMap id="personWithOrdersResultMap" type="Person">
    <id property="id" column="person_id" />
    <result property="name" column="person_name" />
    <collection property="orders" ofType="Order">
        <id property="id" column="order_id" />
        <result property="amount" column="order_amount" />
    </collection>
</resultMap>

这个例子使用左连接在同一个查询中获取Person和Order的数据。

使用鉴别器：

可以结合<discriminator>来使用，用于根据某一列的值动态选择子映射。
适用于需要根据特定字段值映射不同类型对象的场景。


<resultMap id="vehicleResultMap" type="Vehicle">
    <id property="id" column="id" />
    <result property="type" column="type" />
    <discriminator column="type" javaType="String">
        <case value="CAR" resultType="Car" />
        <case value="TRUCK" resultType="Truck" />
    </discriminator>
    <collection property="features" ofType="Feature">
        <id property="id" column="feature_id" />
        <result property="description" column="description" />
    </collection>
</resultMap>
<resultMap id="vehicleResultMap" type="Vehicle">
    <id property="id" column="id" />
    <result property="type" column="type" />
    <discriminator column="type" javaType="String">
        <case value="CAR" resultType="Car" />
        <case value="TRUCK" resultType="Truck" />
    </discriminator>
    <collection property="features" ofType="Feature">
        <id property="id" column="feature_id" />
        <result property="description" column="description" />
    </collection>
</resultMap>
<resultMap id="vehicleResultMap" type="Vehicle">
    <id property="id" column="id" />
    <result property="type" column="type" />
    <discriminator column="type" javaType="String">
        <case value="CAR" resultType="Car" />
        <case value="TRUCK" resultType="Truck" />
    </discriminator>
    <collection property="features" ofType="Feature">
        <id property="id" column="feature_id" />
        <result property="description" column="description" />
    </collection>
</resultMap>