mongodb的aggregate学习之1-pipeline

想知道某个country和province下userid的数量，相同userid算一个

如果这么写，      

[      

{
		$group:
			{
				"_id": {"country": "$country", "province": "$province"},
				'count': {$sum: 1}
			}
	}      

]
      

返回了      

{"result":[{"_id":{"country":"us","province":"shandong"},"count":2},      

{"_id":{"country":"cha","province":"beij"},"count":1},      

{"_id":{"country":"cha","province":"shandong"},"count":3}]      

}
      

从结果来看，计算结果是按照country和province分组了，就是同一个country和province下有几条数据，      

不是我们想要的结果，再改      

[{
		$group:
			{
				"_id": {"country": "$country", "province": "$province", "uid": "$userid"},
				'count': {$sum: 1}
			}
	}]
      

返回了      

{"result":[{"_id":{"country":"cha","province":"beij","uid":"aaa"},"count":1},      

{"_id":{"country":"us","province":"shandong","uid":"aaa"},"count":2},      

{"_id":{"country":"cha","province":"shandong","uid":"bbb"},"count":2},      

{"_id":{"country":"cha","province":"shandong","uid":"aaa"},"count":1}      

]}
      

再改，      

[
	{
		$group:
			{
				"_id": {"country": "$country", "province": "$province", "uid": "$userid"},
				'count': {$sum: 1}
			}
	},
	{
		$group:
			{
				"_id": {"country": "$_id.country", "province": "$_id.province", "uid": "$_id.uid"},
				'count': {$sum: 1}
			}
	}
];
      

返回了      

{"result":[{"_id":{"country":"cha","province":"shandong","uid":"aaa"},"count":1},      

{"_id":{"country":"cha","province":"shandong","uid":"bbb"},"count":1},      

{"_id":{"country":"us","province":"shandong","uid":"aaa"},"count":1},      

{"_id":{"country":"cha","province":"beij","uid":"aaa"},"count":1}      

]}
      

和上一个的结果相比，就是count都变成了1，还不是想要的结果，再改改	      

[
	{
		$group:
			{
				"_id": {"country": "$country", "province": "$province", "uid": "$userid"},
				'count': {$sum: 1}
			}
	},
	{
		$group:
			{
				"_id": {"country": "$_id.country", "province": "$_id.province", /*"uid": "$_id.uid"*/},
				'count': {$sum: 1}
			}
	}
];
      

就是注释了一点点的代码，返回结果      

{"result":[{"_id":{"country":"us","province":"shandong"},"count":1},      

{"_id":{"country":"cha","province":"shandong"},"count":2},      

{"_id":{"country":"cha","province":"beij"},"count":1}]}
      

是想要的结果了