Database
[{"mainCurrency":"USD","fxRate":{"CAD":{"rate":1.35},"GBP":{"rate":0.78},"EUR":{"rate":0.91}}}]
Query
db.collection.update({},[{"$set":{"fxRate":{"$map":{"input":{"$objectToArray":"$fxRate"},"as":"kv","in":{currency:"$$kv.k",rate:"$$kv.v.rate"}}}}}])