本文介绍了首先找到一个字段的每个不同值的文档的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
对于具有字段field1,field2,field3等的文档集合,我需要查找
With a collection of documents with fields field1, field2, field3 and so on, I need to find
- field3 的不同值
- 对于每个field3的不同值,需要获取具有field3中每个不同值的第一个文档
- distinct values for field3
- for each distinct value of field3, need to get the first document with each distinct value in field3
对于#1,我可以做db.myCollection.distinct("field3")
我该如何处理#2?
样品收集:
[ { "field1": 11, "field2": "toyota", "field3": "camry" }, { "field1": 22, "field2": "toyota", "field3": "corolla" }, { "field1": 33, "field2": "toyota", "field3": "camry" }, { "field1": 44, "field2": "honda", "field3": "accord" }, { "field1": 55, "field2": "honda", "field3": "accord" }, { "field1": 66, "field2": "honda", "field3": "city" } ]
所需结果:
[ { "field1": 11, "field2": "toyota", "field3": "camry" }, { "field1": 22, "field2": "toyota", "field3": "corolla" }, { "field1": 44, "field2": "honda", "field3": "accord" }, { "field1": 66, "field2": "honda", "field3": "city" } ]
推荐答案
您需要运行一个汇总操作,该操作按field3对所有文档进行分组,并使用 $first 累加器,其中包含 $$ROOT 系统变量来引入第一个文档,例如以下:
You need to run an aggregate operation that groups all the documents by field3 and use the $first accumulator with the $$ROOT system variable to bring the first document, something like the following:
db.myCollection.aggregate([ { "$group": { "_id": "$field3", "doc": { "$first": "$$ROOT" } } } ])
或获取准确的输出:
db.myCollection.aggregate([ { "$group": { "_id": "$field3", "field1": { "$first": "$field1" }, "field2": { "$first": "$field2" } } }, { "$project": { "_id": 0, "field3": "$_id", "field2": 1, "field1": 1 } } ])
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