[{"receiverWorkout":{"name":"nmbnmbnmnbm","exercises":[{"_id":"1","reps":0},{"_id":"3","reps":4}]},"senderWorkout":{"name":"nmbnmbnmnbm","exercises":[{"_id":"2","reps":12},{"_id":"1","reps":8}]}}]

db.collection.aggregate([{$addFields:{"senderWorkout.exercises":{$map:{input:"$senderWorkout.exercises",as:"ex",in:{$cond:{if:{$in:["$$ex._id","$receiverWorkout.exercises._id"]},then:{_id:"$$ex._id",reps:"$$ex.reps",result:true},else:{_id:"$$ex._id",reps:"$$ex.reps",result:false}}}}}}}])