db={"Faults":[{"_id":"99812930-37CE-456F-A9D9-837E9E3F712A","faultsChanged":[{"_id":"7C628A46-7E80-4615-8B08-10C5E9A6B1D7","faults":["BF221A71-0217-42E7-B853-53112EDA9694","E4A54172-7E93-49C4-840B-8E6116116979"],"isDeleted":false,"partition":"indego","sessionUuid":"A83CE9A1-7539-493F-8BA4-6FBE25B18B57","source":"1","timestamp":{"$date":{"$numberLong":"1630603342700"}},"unmigratedNote":null,"uuid":"7C628A46-7E80-4615-8B08-10C5E9A6B1D7"}]}],"00FaultEntry":[{"_id":"BF221A71-0217-42E7-B853-53112EDA9694","some_key":"value"},{"_id":"E4A54172-7E93-49C4-840B-8E6116116979","some_key":"value"}]}

db.Faults.aggregate([{"$unwind":"$faultsChanged"},{"$lookup":{"from":"00FaultEntry","localField":"faultsChanged.faults","foreignField":"_id","as":"faultsChanged.faults"}},{"$group":{"_id":"$_id","faultsChanged":{$push:"$faultsChanged"}}}])