db={"users":[{"name":"John","available":true,"province":"Rome","city":"Tivoli"},{"name":"Difool","available":true,"province":"Rome","city":"Ostia"},{"name":"Paul","available":true,"province":"Rome","city":"Rome"},{"name":"Andrew","available":false,"province":"Rome","city":"Grottaferrata"}],"inventory":[{"start_province":"Rome","start_city":"Rome","end_province":"Rome","end_city":"Ostia","distance":5},{"start_province":"Rome","start_city":"Rome","end_province":"Rome","end_city":"Tivoli","distance":8}]}

db.users.aggregate([{"$match":{"available":{"$eq":true}}},{"$lookup":{"from":"distances","let":{"uprovince":"$province","ucity":"$city"},"pipeline":[{"$match":{"$expr":{"$and":[{"$eq":["$$uprovince","$end_province"]},{"$eq":["$$ucity","$end_city"]}]}}}],"as":"distances"}},{"$set":{"distance":{"$cond":[{"$eq":["$distances",[]]},0,{"$arrayElemAt":["$distances.distance",0]}]}}},{"$unset":["distances"]}])