[{"id":1,"array":[{"code":"a"},{"code":"b"}]}]

db.collection.update({"id":3},[{$set:{array:{$cond:[{$in:["a",{$ifNull:["$array.code",[]]}]},{$map:{input:"$array",in:{$mergeObjects:["$$this",{$cond:[{$eq:["$$this.code","a"]},{test:"ok"},{}]}]}}},{$concatArrays:[{$ifNull:["$array",[]]},[{test:"ok",code:"a"}]]}]}}}],{upsert:true})