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模板：
<!-- ？属性，本类与？类的多对多的关系 -->
<set name="第一个问号值" table="中间表（类名_对方类名 或 对方类名_类名）其中两个地方的要一样">
<key column=" （本类的名字+Id） "></key>
<many-to-many class="第二个问号值" column="（对方的key里面的column属性的值）"></many-to-many>
</set>
<!-- ？属性，本类与？类的多对一的关系 -->
<many-to-one name="（第一个问号的值）" class="（第二个问号的值）" column="（name的属性名 + Id）"></many-to-one>
<!-- ？属性，本类与？类的一对多 -->
<set name="（第一个问号的值）">
<key column="（与其关联（多对一中）的类的column的值） "></key>
<one-to-many class="（第二个问号的值）" />
</set>
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eg：
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<!-- department属性，本类与Department的多对一 -->
<many-to-one name="department" class="Department" column="departmentId"></many-to-one>
<!-- roles属性，本类与Role的多对多 -->
<set name="roles" table="itcast_user_role">
<key column="userId"></key>
<many-to-many class="Role" column="roleId"></many-to-many>
</set>
<!-- users属性，本类与User的一对多 -->
<set name="users">
<key column="departmentId"></key>
<one-to-many class="User" />
</set>
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---------------------------------------------------------------------------------------------------
<!-- parent属性，本类与Department（上级）的多对一 -->
<many-to-one name="parent" class="Department" column="parentId"></many-to-one>
<!-- children属性，本类与Department（下级）的一对多 -->
<set name="children">
<key column="parentId"></key>
<one-to-many class="Department" />
</set>
---------------------------------------------------------------------------------
---------------------------------------------------------------------------------
<!-- users属性，本类与User的多对多 -->
<set name="users" table="itcast_user_role">
<key column="roleId"></key>
<many-to-many class="User" column="userId"></many-to-many>
</set>
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