MongoDB:计算数组中的项目数 [英] MongoDB: count the number of items in an array

问题描述

我有一个集合，集合中的每个文档都有一个名为foo的数组，该数组包含一组嵌入式文档. MongoDB Shell中当前是否有一种简单的方法来计算foo中有多少个实例?像这样:

I have a collection where every document in the collection has an array named foo that contains a set of embedded documents. Is there currently a trivial way in the MongoDB shell to count how many instances are within foo? something like:

db.mycollection.foos.count()或db.mycollection.foos.size()?

数组中的每个文档都必须具有唯一的foo_id，我想进行一次快速计数，以确保集合中随机文档的数组中包含正确数量的元素.

Each document in the array needs to have a unique foo_id and I want to do a quick count to make sure that the right amount of elements are inside of an array for a random document in the collection.

推荐答案

在MongoDB 2.6中，聚合框架具有一个新数组

In MongoDB 2.6, the Aggregation Framework has a new array $size operator you can use:

> db.mycollection.insert({'foo':[1,2,3,4]})
> db.mycollection.insert({'foo':[5,6,7]})

> db.mycollection.aggregate({$project: { count: { $size:"$foo" }}})
{ "_id" : ObjectId("5314b5c360477752b449eedf"), "count" : 4 }
{ "_id" : ObjectId("5314b5c860477752b449eee0"), "count" : 3 }

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