db={"col1":[{"field1":"value1","mycode":"1234","field3":"value3"}],"col2":[{"code":"1234","label":"myLabel"}]}

db.col1.aggregate([{"$lookup":{"from":"col2","localField":"mycode","foreignField":"code","as":"mycode"}},{"$unwind":"$mycode"},{"$project":{"field1":1,"field3":1,"_id":0,"mycode":"$mycode.label"}}])