[{productName:"ABC",price:{previous:[null,null,100],latest:[200,200,200]}},{productName:"ABC",price:{previous:[null,null,null],latest:[200,200,200]}},{productName:"ABC",price:{previous:[null,null,0],latest:[200,200,200]}},{productName:"ABC",price:{previous:[1,null],latest:[100,null]}}]

db.collection.update({},[{$set:{discount:{$cond:{if:{$and:[{$gt:[{$last:"$price.latest"},0]},{$gt:[{$last:"$price.previous"},0]}]},then:{$divide:[{$last:"$price.latest"},{$last:"$price.previous"}]},else:null,}}}}],{multi:true})