本文介绍了MongoDB按引用的文档属性计数的处理方法,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
db.foos
{
bar: ObjectId('123')
}
db.bars
{
_id: ObjectId('123')
type: 'wine'
}
如何以最简单的方式找到引用葡萄酒"类型的bar文档的foo文档数量?希望即使集合中包含大量文档,也可以进行缩放以使其表现良好.
How can I in the simplest way find the number of foo-documents that refers to a bar-document of type 'wine'? Hopefully one that scales to perform fairly well even if the collections should contain a very large number of documents.
推荐答案
尝试以下聚合框架查询:
Try this aggregation framework query:
db.foos.aggregate([
{$lookup:
{
from: "bars",
localField: "_id",
foreignField: "_id",
as: "docs"
}
},
{$unwind: "$docs"},
{$match: {"docs.type":"wine"}},
{$group: {"_id":"$_id", count: {$sum:1}}}
]
)
我在这些文档上对其进行了测试:
I tested it on these documents:
db.foos.insert({"_id":"123"})
db.foos.insert({"_id":"456"})
db.bars.insert({"_id":"123", type:"wine"})
db.bars.insert({"_id":"456", type:"beer"})
对于葡萄酒类型,我得到的结果是:
and for wine type I get as result:
{
"_id" : "123",
"count" : 1
}
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