[{type:"apple",price:10},{type:"apple",price:5},{type:"apple",price:3},{type:"orange",price:2},{type:"pear",price:12},{type:"pear",price:2}]

db.collection.aggregate([{$match:{type:{$in:["apple","orange","pear"]}}},{$group:{_id:"$type",price:{$max:"$price"}}},{$project:{type:"$_id",_id:0,price:1}}])