[{"postType":"buy","product":"3",},{"postType":"view","product":"4",},{"postType":"buy","product":"2",},{"postType":"view","product":"2",},{"postType":"share","product":"3",},{"postType":"buy","product":"2",},{"postType":"buy","product":"2",},{"postType":"share","product":"2",},{"postType":"buy","product":"1",},{"postType":"buy","product":"1",},{"postType":"viewvideo","product":"1",},{"postType":"viewvideo","product":"2",},{"postType":"viewvideo","product":"3",},{"postType":"buy","product":"4",},{"postType":"buy","product":"5",}]

db.collection.aggregate([{$match:{postType:"buy"}},{$group:{_id:"$product",count:{$count:{}}}},{$project:{product:"$_id",count:1,_id:0}}])