[{"item":"journal","instock":[{"warehouse":"A","ele":[2,4,5]},{"warehouse":"C","ele":[8,5,2]},{"warehouse":"F","ele":[3]},{"warehouse":"K","ele":[2,8,4]}]}]

db.collection.aggregate([{"$unwind":{path:"$instock",preserveNullAndEmptyArrays:true}},{"$match":{"instock.ele":{$eq:5}}},{"$group":{"_id":"item","instock":{"$push":"$instock"}}}])