collection 的 ''permissions" 是map 的key,通过key取出对应的list,
item是遍历list中当前的对象,item.id 是取出对象的id
//这个list的结构
permissions=[Permission{name='null', resource='null', id=4}], id=1}
<insert id="savePermissions" parameterType="hashmap" >
insert into t_role_permission(role_id,permission_id) values
<foreach collection="permissions" item="item" separator=",">
(#{id},#{item.id})
</foreach>
</insert>