集合的元素基本上应该是三层的嵌套吧
{"province":{"pname":p1,"city":[{"cname":c11,"area":[{"aname":a11}]}]}}
{"province":{"pname":p2,"city":[{"cname":c21,"area":[{"aname":a21}]}]}}
{"province":{"pname":p3,"city":[{"cname":c31,"area":[{"aname":a31}]}]}}
使用时根据province检索省的名称,然后将内嵌在province document中的city(array)迭代,获取名称,和内嵌在每个city document中的area(area)迭代,获取所有区名称