[{_id:1,product:"Book",quantity:5,price:10,category:"stationeries"},{_id:2,product:"Pen",quantity:2,price:2,category:"stationeries"},{_id:3,product:"Ruler",quantity:1,price:4,category:"stationeries"},{_id:4,product:"Carrot",quantity:1,price:8,category:"vegetable"},{_id:5,product:"Cabbage",quantity:2,price:12,category:"vegetable"},{_id:6,product:"Lettuce",quantity:1,price:18,category:"vegetable"}]

db.collection.aggregate([{$group:{_id:"$category",firstProduct:{$first:"$product"}}}])