Find all documents that share max(value) found in aggregate step

问题

I'm trying to find all documents that share the max(num_sold). I cant just sort descending and do a limit(1) because I'd miss the case where other docs also have the same max(num_sold). Given this over-simplified dataset:

{"item":"Apple", "num_sold": 49}
{"item":"Orange", "num_sold": 55}
{"item":"Peach", "num_sold": 55}
{"item":"Grape", "num_sold": 20}
{"item":"Banana", "num_sold": 20}

I want to get back

{"item":"Orange", "num_sold": 55}
{"item":"Peach", "num_sold": 55}    

Using SQL, this is a simple query and then join back to the main dataset. I'm a bit stumped on how to do this in MongoDB.

I've found the max, but how do I get all docs that have that max?

db.t.aggregate(
    {$group:{
          _id:null,
          num_sold:{$max:"$num_sold"}
          }
    }
);
{ "_id" : null, "num_sold" : 55 }

回答1:

You can do this by grouping on num_sold and then using $sort and $limit pipeline stages to get just the docs with the maximum value:

db.t.aggregate([
    // Group by num_sold, assembling an array of docs with each distinct value.
    {$group: {
        _id: '$num_sold',
        docs: {$push: '$$ROOT'}
    }},
    // Sort the groups by _id descending to put the max num_sold group first.
    {$sort: {_id: -1}},
    // Return just the first (max num_sold) group of docs.
    {$limit: 1}
])

Output:

{ 
    "_id" : 55.0, 
    "docs" : [
        {
            "_id" : ObjectId("5726a62879ce3350ff8d607e"), 
            "item" : "Orange", 
            "num_sold" : 55.0
        }, 
        {
            "_id" : ObjectId("5726a62879ce3350ff8d607f"), 
            "item" : "Peach", 
            "num_sold" : 55.0
        }
    ]
}

来源：https://stackoverflow.com/questions/36973145/find-all-documents-that-share-maxvalue-found-in-aggregate-step