[{"_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([{$project:{properties:{$reduce:{input:"$config",initialValue:[],in:{$concatArrays:["$$value",{$map:{input:{$objectToArray:"$$this"},in:"$$this.k"}}]}}}}}])