使用mongo聚合框架按数组的特定元素分组 [英] Group by specific element of array with mongo aggregation framework

问题描述

是否可以使用聚合框架对数组的特定元素进行分组?

Is it possible to use the aggregation framework to group by a specific element of an array?

诸如此类的文件:

{
  name: 'Russell',
  favourite_foods: [
    { name: 'Pizza', type: 'Four Cheeses' },
    { name: 'Burger', type: 'Veggie'}
  ],
  height: 6
}

我可以获得最喜欢的食物(即索引为0的食物)的不同列表，以及最喜欢的食物的最高者的身高是什么?

I could get a distinct list of top favourite foods (ie. foods at index 0) along with the height of the tallest person who's top favourite food that is?

像这样的事情(尽管由于数组索引访问点符号在聚合框架中似乎不起作用，所以它不起作用):

Something like this (although it doesn't work as the array index access dot notation doesn't seem to work in the aggregation framework):

db.people.aggregate([
  { $group : { _id: "$favourite_foods.0.name", max_height: { $max : "$height" } } }
])

推荐答案

似乎您要依靠数组中每个人最喜欢的食物.如果是这样，您可以利用聚合框架运算符.

Seems like you are relying on the favorite food for each person being first in the array. If so, there is an aggregation framework operator you can take advantage of.

这是您可以使用的管道:

Here is the pipeline you can use:

db.people.aggregate(
[
    {
        "$unwind" : "$favourite_foods"
    },
    {
        "$group" : {
            "_id" : {
                "name" : "$name",
                "height" : "$height"
            },
            "faveFood" : {
                "$first" : "$favourite_foods"
            }
        }
    },
    {
        "$group" : {
            "_id" : "$faveFood.name",
            "height" : {
                "$max" : "$_id.height"
            }
        }
    }
])

在此样本数据集上:

> db.people.find().pretty()
{
    "_id" : ObjectId("508894efd4197aa2b9490741"),
    "name" : "Russell",
    "favourite_foods" : [
        {
            "name" : "Pizza",
            "type" : "Four Cheeses"
        },
        {
            "name" : "Burger",
            "type" : "Veggie"
        }
    ],
    "height" : 6
}
{
    "_id" : ObjectId("5088950bd4197aa2b9490742"),
    "name" : "Lucy",
    "favourite_foods" : [
        {
            "name" : "Pasta",
            "type" : "Four Cheeses"
        },
        {
            "name" : "Burger",
            "type" : "Veggie"
        }
    ],
    "height" : 5.5
}
{
    "_id" : ObjectId("5088951dd4197aa2b9490743"),
    "name" : "Landy",
    "favourite_foods" : [
        {
            "name" : "Pizza",
            "type" : "Four Cheeses"
        },
        {
            "name" : "Pizza",
            "type" : "Veggie"
        }
    ],
    "height" : 5
}
{
    "_id" : ObjectId("50889541d4197aa2b9490744"),
    "name" : "Augie",
    "favourite_foods" : [
        {
            "name" : "Sushi",
            "type" : "Four Cheeses"
        },
        {
            "name" : "Pizza",
            "type" : "Veggie"
        }
    ],
    "height" : 6.2
}

您将获得以下结果:

{
    "result" : [
        {
            "_id" : "Pasta",
            "height" : 5.5
        },
        {
            "_id" : "Pizza",
            "height" : 6
        },
        {
            "_id" : "Sushi",
            "height" : 6.2
        }
    ],
    "ok" : 1
}

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