[{"_id":123456,"Item_Data":{payload:{items:[{key1:"string1",key2:"string2",key3:"string3",key4:"string4",},{key1:"string5",key2:"string6",key3:"string7",},{key1:"string8",key2:"string9",}]}}},{"_id":654321,"Item_Data":{payload:{items:[{key1:"string11",key2:"string12",key3:"string13",key4:"string14",},{key1:"string15",key2:"string16",key3:"string17",},{key1:"string18",key2:"string19",}]}}},]

db.collection.aggregate([{$match:{"Item_Data.payload.items.key1":"string1"},},{$unwind:"$Item_Data.payload.items"},{$match:{"Item_Data.payload.items.key1":"string1"},},{$project:{_id:1,key2:"$Item_Data.payload.items.key2"}}])