使用mgo精确到小数

Question

I am writing an application where I use money and want very accurate numbers. I am also using mgo to store the results after some application. I was wondering if there was a way for me to use math.Rat or godec in a struct and have it store as a number in mgo?

This is the kind of code i was hoping to run:

package main

import(
  "fmt"
  "math/big"
  "labix.org/v2/mgo"
)

var mgoSession *mgo.Session

type Test struct{
  Budget big.Rat
}

func MongoLog(table string, pointer interface{}) {
  err := mgoSession.DB("db_log").C(table).Insert(pointer)
  if err != nil {
    panic(err)
  }
}

func main(){
  var err error
  mgoSession, err = mgo.Dial("localhost:27017")
  defer mgoSession.Close()
  if err != nil {
    panic(err)
  }

  cmp := big.NewRat(1, 100000)
  var test = Test{Budget : *big.NewRat(5, 10)}
  MongoLog("test", &test)
  for i := 0; i < 20; i++{
    fmt.Printf("Printf: %s
", test.Budget.FloatString(10))
    fmt.Println("Println:", test.Budget, "
")
    test.Budget.Sub(&test.Budget, cmp)
//    test.Budget = test.Budget - cpm
  }
  MongoLog("test", &test)
}

Answer 1

big.Rat is basically a pair of unexported int big.Int values describing the numerator and denominator of a rational number, respectively.

You can easily get both numbers through (*big.Rat).Denom and (*big.Rat).Num.

Then store them in a structure of your own, with exported (upper case) fields:

type CurrencyValue struct {
    Denom int64
    Num   int64
}

Store this with mgo and convert it back to a *big.Rat in your application through big.NewRat

Edit:

Nick Craig-Wood in the comments correctly noted that big.Rat actually consists of 2 big.Int values, not int values as I had written (easy to miss the upper case i). It's hard to represent a big.Int in BSON but, int64 should cover most use-cases.