MyBatis：ResultMap的继承

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当数据实体具有一对多，或多对多的关系时，如果需要分别编写级联获取，非级联获取的接口，为了避免定义多了ResultMap，可以使用ResultMap的extends属性来优化。

优化前：

<resultMap type="com.bean.Topology" id="resultMapTopology">
    <id column="topology_pk" property="topology_pk" javaType="java.lang.Long"/>	
    <result column="topology_id" property="topology_id" javaType="java.lang.String"/>
</resultMap>

<resultMap type="com.bean.Topology" id="resultMapTopologyOnConnection">
    <id column="topology_pk" property="topology_pk" javaType="java.lang.Long"/>	
    <result column="topology_id" property="topology_id" javaType="java.lang.String"/>
		
    <collection 
        property="topology_hostlist" 
        column="topology_pk"             
        select="com.mapper.HostMapper.getHostListByTopology_Pk" />
</resultMap>

<!-- 获取拓扑的全量数据 （非级联获取，不传参） -->
<select id="getTopologyList" resultMap="resultMapTopology">
	select * from odl_table_topology
</select>
	
<!-- 获取拓扑的全量数据 （级联获取，不传参）  -->
<select id="getTopologyListOnConnection" resultMap="resultMapTopologyOnConnection">
	select * from odl_table_topology
</select>

优化后：

<resultMap type="com.bean.Topology" id="resultMapTopology">
    <id column="topology_pk" property="topology_pk" javaType="java.lang.Long"/>	
    <result column="topology_id" property="topology_id" javaType="java.lang.String"/>
</resultMap>

<resultMap type="com.bean.Topology" id="resultMapTopologyOnConnection" extends="resultMapTopology">
    <collection 
        property="topology_hostlist" 
        column="topology_pk"             
        select="com.mapper.HostMapper.getHostListByTopology_Pk" />
</resultMap>

<!-- 获取拓扑的全量数据 （非级联获取，不传参） -->
<select id="getTopologyList" resultMap="resultMapTopology">
	select * from odl_table_topology
</select>
	
<!-- 获取拓扑的全量数据 （级联获取，不传参）  -->
<select id="getTopologyListOnConnection" resultMap="resultMapTopologyOnConnection">
	select * from odl_table_topology
</select>