db={"users":[{"_id":1,"name":"Sam","tier":1},{"_id":2,"name":"Tom","tier":2}],"tiers":[{"_id":1,"tier":"bronze","cost":0},{"_id":2,"tier":"silver","cost":100},{"_id":3,"tier":"gold","cost":200}]}

db.users.aggregate([{"$lookup":{"from":"tiers","localField":"tier","foreignField":"_id","as":"tiers"}}])