[{_id:1,"firstName":"ABC","lastName":"DEF","payItems":[{"ACId":{"$oid":"6168825c5d0edbc0b61615db"},"balance":0,"rate":0},{"ACId":{"$oid":"6168825c5d0edbc0b61615dc"},"balance":1,"rate":2}]}]

db.collection.aggregate([{"$match":{"_id":1}},{"$group":{"_id":null,"payItems":{"$push":"$payItems.ACId"}}},{"$project":{"_id":0,"payItems":1}},{$unwind:"$payItems"}])