db={Students:[{"id":"123","title":"John","marks_in_subjects":[{"marks":90,"subject_id":"abc"},{"marks":92,"subject_id":"def"}]}],Subjects:[{"id":"abc","name":"math"},{"id":"def","name":"physics"}]}

db.Students.aggregate([{"$lookup":{"from":"Subjects","localField":"marks_in_subjects.subject_id","foreignField":"id","as":"subjects"}},{$project:{subjects:"$subjects.name",title:1}},{"$addFields":{titleAll:{$map:{input:"$subjects",as:"item",in:{$concat:["$title"," > ","$$item"]}}}}},{$project:{title:"$title",titleAll:{"$concatArrays":["$titleAll",["$title"]]}}},{$unwind:"$titleAll"}])