[{"elements":{"title":"elem_1_6_title","sub_title":"elem_1_6_sub_title","media":"elem_1_6_media","button":"elem_1_6_button","give_us_call":"elem_1_6_give_us_call"}},{"elements":{"title":"elem_1_6_title","sub_title":"elem_1_6_sub_title"}},{"elements":{}}]

db.collection.aggregate([{$addFields:{elements:{"$objectToArray":"$elements"}}},{"$addFields":{"elements":{"$arrayToObject":{"$map":{"input":"$elements","as":"e","in":{k:"$$e.v",v:"$$e.k"}}}}}}])