db={"contracts":[{"bid":1,"type":"A","name":"N1"},{"bid":1,"type":"B","name":"N2"},{"bid":1,"type":"C","name":"N3"}],"businesses":[{"id":1,"contract_settings":{"A":{"price":100},"B":{"price":200},"default":"A"}}]}

db.contracts.aggregate([{$lookup:{from:"businesses",localField:"bid",foreignField:"id",as:"businesses"}}])