[{"_id":0,"config":[{"property1":"a","property2":"b","property3":"c","property4":"d"},{"property1":"a","property2":"bb","property3":"cc","property4":"d","ispropert5":true},{"property1":"a","property2":"b","property3":"c","property4":"ddd","ispropert5":false,"ispropert6":false}],"entity":"123asdf","url":"","createdDate":1}]

db.collection.aggregate([{"$project":{"_id":0,keys:{"$map":{"input":{"$objectToArray":{$reduce:{input:"$config",initialValue:{},in:{"$mergeObjects":"$$this"}}}},"in":"$$this.k"}}}}])