MongoDB query for all documents with unique field

mongodb

Solution

> 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"
    }
]

Problem

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

Original source