[{"course_id":"c1","language":"french","topics":[{"topic_id":"t1","topic_title":"one","topic_description":"one topic_description"},{"topic_id":"t2","topic_title":"two","topic_description":"two topic_description"}]}]

db.collection.aggregate([{$match:{course_id:"c1",language:"french"}},{$unwind:"$topics"},{$replaceRoot:{newRoot:"$topics"}},{$project:{topic_id:1,topic_title:1}}])