[{_id:"123",name:"product1",quantity:10},{_id:"456",name:"product2",quantity:20},{_id:"789",name:"product1",quantity:30},{_id:"145",name:"product3",quantity:30}]

db.collection.aggregate([{"$match":{name:{$in:["product1","product2"]}}},{$group:{_id:"$name",quantity:{$sum:"$quantity"}}}])