MongoDB查询具有唯一字段的所有文档

问题描述：

我对如何在一个字段中获得所有唯一值"这个问题有很多回答.建议使用.distinct()方法.但这将返回这些值的简单数组.如何检索所有具有字段唯一值的文档?

I see plenty of responses to the question "how do I get all of the unique values in a field?" which suggest the .distinct() method. But this returns a simple array of those values. How do I retrieve all of the documents which HAVE a unique value of a field?

[{age: 21, name: 'bob'}, {age: 21, name: 'sally'}, {age: 30, name: 'Jim'}] 
Query for unique age -->
[{age: 21, name: 'sally'}, {age: 30, name: 'Jim'}]
or
[{age: 21, name: 'bob'}, {age: 30, name: 'Jim'}]

事后过滤查询不是一个理想的解决方案，因为我仍然会像往常一样选择$ limit和$ skip.

Filtering a query after-the-fact is not an ideal solution, as I will still want to select, $limit, and $skip as usual.

答

> db.foo.insert([{age: 21, name: 'bob'}, {age: 21, name: 'sally'}, {age: 30, name: 'Jim'}])
> db.foo.count()
3
> db.foo.aggregate({ $group: { _id: '$age', name: { $max: '$name' } } }).result
[
    {
        "_id" : 30,
        "name" : "Jim"
    },
    {
        "_id" : 21,
        "name" : "sally"
    }
]

MongoDB查询具有唯一字段的所有文档

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