MongoDB查询具有唯一字段的所有文档

Question

我看到很多回答"如何在一个领域获得所有独特价值？"的问题.这表明了这种.distinct()方法.但是这会返回这些值的简单数组.如何检索具有字段唯一值的所有文档？

[{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.

Answer 1

> 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"
    }
]