[{_id:ObjectId("619365c54a7d53716438933e"),name:"Chris",hobbies:[{title:"Sports",frequency:5},{title:"Cooking",fequency:5},{title:"Hiking",frequency:1}],isSporty:true,cookMaster:true,age:35}]

db.collection.update({},[{"$set":{"hobbies":{"$map":{"input":"$hobbies","as":"h","in":{title:"$$h.title",frequency:{"$ifNull":["$$h.frequency","$$h.fequency"]}}}}}}])