[{"_id":ObjectId("5f8069b848e54248f8302b18"),"topics":[{"_id":"123","name":"ABC","type":0},{"_id":"455","name":"DFR","type":0}],"topic":{"_id":"777","name":"FFG","type":123}}]

db.collection.aggregate([{"$project":{topics:{"$concatArrays":["$topics",["$topic"]]}}}])