Database
[{productName:"ABC",category:"cat1"},{productName:"DEF",category:"cat2"},{productName:"GHI",category:"cat3"},{productName:"JKL",category:"cat1"},{productName:"MNO",category:"cat2"},{productName:"PQR",category:"cat3"}]
Query
db.collection.aggregate([{$group:{_id:"$category",products:{$push:{"productName":"$productName"}}}}])