Database
[{"date":"Mon Dec 27 2010 18:51:22 GMT+0000 (UTC)",stats:{"floor_price":15.20}},{"date":"Mon Dec 27 2010 18:51:23 GMT+0000 (UTC)",stats:{"floor_price":15.20}},{"date":"Mon Dec 27 2010 18:51:23 GMT+0000 (UTC)",stats:{"floor_price":15.20}},{"date":"Mon Dec 27 2010 18:51:23 GMT+0000 (UTC)",stats:{"floor_price":15.21}}]
Query
db.collection.aggregate([{$group:{_id:"$stats.floor_price",date:{$max:"$date"},count:{$sum:1}}},{$sort:{"date":1}}])