[{nId:0,children0:[{nId:3,access:[{permission:"view",email:"user1@email.com"}]},{nId:4,access:[{permission:"view",email:"user2@email.com"}]}]}]

db.collection.aggregate([{"$set":{"birthdayservice":{"$map":{"input":"$children0","in":{"$mergeObjects":["$$bs",{"access":{"$map":{"input":"$$bs.email","in":{"$mergeObjects":["$$t",{"hasAccess":true}]},"as":"t"}}}]},"as":"bs"}}}}])