db={"Interest":[{"CompanyNumber":"09145694","Data":{"CountryOfResidence":"England","DateOfBirth":{"Month":2,"Year":1977},"Name":"Mrs Nga Thanh Wildman","NameElements":{"Forename":"Nga","MiddleName":"Thanh","Surname":"Wildman","Title":"Mrs"}}},{"CompanyNumber":"94","Data":{"CountryOfResidence":"England","DateOfBirth":{"Month":4,"Year":1980},"Name":"Mr John Glen","NameElements":{"Forename":"John","MiddleName":"Thanh","Title":"Mr"}}}],"People":[{"Name":"Mrs Nga Thanh Wildman","BirthMonth":2,"BirthYear":1977}]}

db.Interest.aggregate([{$project:{Name:"$Data.Name",BirthMonth:"$Data.DateOfBirth.Month",BirthYear:"$Data.DateOfBirth.Year",_id:0}},{$merge:{into:"People",on:["Name","BirthMonth","BirthYear"]}}])