[{zip:94301,name:"Palo Alto"},{zip:10019,name:"New York"}]

db.collection.aggregate([{$lookup:{localField:"zip",foreignField:"zip_id",as:"city_state",pipeline:[{$documents:{$concatArrays:[[{zip_id:94301,name:"Palo Alto, CA"}],[{zip_id:10019,name:"New York, NY"}]]}}]}}])