首先找到一个字段的每个不同值的文档 [英] find first of documents for each distinct values for one field

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
1. 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

1. distinct values for field3
2. 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

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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