如何将Golang big.Int存储到MongoDB中

Question

I have a struct referencing a *big.Int. When storing this struct naively into MongoDB (using the official driver) the field turns to be nil when fetching the struct back. What is the proper/best way to store a big.Int into MongoDB?

    type MyStruct struct {
        Number *big.Int
    }

    nb := MyStruct{Number: big.NewInt(42)}
    r, _ := db.Collection("test").InsertOne(context.TODO(), nb)

    result := &MyStruct{}
    db.Collection("test").FindOne(context.TODO(), bson.D{{"_id", r.InsertedID}}).Decode(result)
    fmt.Println(result) // <== Number will be 0 here

My best idea so far would be to create a wrapper around big.Int that implements MarshalBSON and UnmarshalBSON (which I am not even sure how to do properly to be honest). But that'd be quite inconvenient.

Answer 1

Here's a possible implementation I came up with that stores the big.Int as plain text into MongoDb. It is also possible to easily store as byte array by using methods Bytes and SetBytes of big.Int instead of MarshalText/UnmarshalText.

package common

import (
    "fmt"
    "math/big"

    "go.mongodb.org/mongo-driver/bson"
)

type BigInt struct {
    i *big.Int
}

func NewBigInt(bigint *big.Int) *BigInt {
    return &BigInt{i: bigint}
}

func (bi *BigInt) Int() *big.Int {
    return bi.i
}
func (bi *BigInt) MarshalBSON() ([]byte, error) {
    txt, err := bi.i.MarshalText()
    if err != nil {
        return nil, err
    }
    a, err := bson.Marshal(map[string]string{"i": string(txt)})
    return a, err
}

func (bi *BigInt) UnmarshalBSON(data []byte) error {
    var d bson.D
    err := bson.Unmarshal(data, &d)
    if err != nil {
        return err
    }
    if v, ok := d.Map()["i"]; ok {
        bi.i = big.NewInt(0)
        return bi.i.UnmarshalText([]byte(v.(string)))
    }
    return fmt.Errorf("key 'i' missing")
}