Return a document with the max value on a number field using MongoDB aggregation

问题

If I have a bunch of documents for instance

{
  _id: mongoId,
  subcatId: mongoId,
  viewCount: 2
}

_id is unique but subcatId isn't.

If I wanted to return each document that had the highest viewCount per subcatId, how would I do that using aggregation with Mongoose/MongoDB?

回答1:

You can do that like this:

db.test.aggregate([
  // Sort the docs by viewCount descending so that the highest ones come first
  {$sort: {viewCount: -1}},
  // Group by subcatId and take the first doc from each group
  {$group: {_id: '$subcatId', doc: {$first: '$$ROOT'}}}
])

The $$ROOT system variable was added in 2.6 and represents the whole document being processed at that stage in the pipeline. All system variables are referenced with the $$ prefix.

For older versions of MongoDB, you need to individually add each field you need to the $group:

db.test.aggregate([
  {$sort: {viewCount: -1}},
  {$group: {
    _id: '$subcatId', 
    doc_id: {$first: '$_id'},
    viewCount: {$first: '$viewCount'}
  }}
])

来源：https://stackoverflow.com/questions/26811178/return-a-document-with-the-max-value-on-a-number-field-using-mongodb-aggregation