Database
[{"id":"yzes547","courseId":"aw4a34y7","objectives":[{"type":"LESSON","id":"dfhxytd"},{"type":"MODULE","id":"rgzeerp"}]}]
Query
db.collection.aggregate([{$replaceRoot:{newRoot:{"$mergeObjects":["$$ROOT",{"$arrayToObject":{$map:{input:"$objectives",as:"obj",in:{k:{$concat:[{"$toLower":"$$obj.type"},"Id"]},v:"$$obj.id"}}}}]}}}])