Database
db={"flight":[{"_id":ObjectId("5c4ae5b6a2ac3bc9f2e1b943"),"verified":"true","valid":"true"},{"_id":ObjectId("5c4ae5eea2ac3bc9f2e1b961"),"verified":"true","valid":"true"}],"mission":[{"_id":ObjectId("5c5acdd8bbf999dc34d96de7"),"flights":[ObjectId("5c4ae5b6a2ac3bc9f2e1b943"),ObjectId("5c4ae5eea2ac3bc9f2e1b961")]}]}
Query
db.mission.aggregate([{$match:{"_id":ObjectId("5c5acdd8bbf999dc34d96de7")}},{$lookup:{from:"flight",let:{flightIds:"$flights"},pipeline:[{$match:{$expr:{$in:["$_id","$$flightIds"]}}},],as:"result"}}])