[{modified:"11/02/2020",stocks:[{product:{name:"Milk",price:20},quantity:2,paid:true},{product:{name:"Milk1",price:20},quantity:2,paid:true}]},{modified:"10/02/2020",stocks:[{product:{name:"Sugar",price:50},quantity:1,paid:false}]},{modified:"10/02/2020",stocks:[{product:{name:"Butter",price:10},quantity:5,paid:false}]}]

db.collection.aggregate([{$unwind:"$stocks"},{$group:{_id:"$modified",records:{$push:"$stocks"}}}])