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按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?

这样的文档:

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

我可以获得一份最喜欢的食物(即索引为 0 的食物)的不同列表，以及最喜欢的食物中最高的人的身高?

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?

类似这样的东西(虽然它不起作用，因为数组索引访问点表示法在聚合框架中似乎不起作用):

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

似乎你在依赖每个人最喜欢的食物排在第一个.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"
            }
        }
    }
])

在这个示例数据集上:

> 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
}

你会得到这些结果:

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

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