How to store Golang big.Int into MongoDB - mongodb

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")
}

Answer 2

the field turns to be nil when fetching the struct back
The reason why it returns 0, is because there is no bson mapping available for big.Int. If you check the document inserted into MongoDB collection you should see something similar to below:
{
"_id": ObjectId("..."),
"number": {}
}
Where there is no value stored in number field.
What is the proper/best way to store a big.Int into MongoDB?
Let's first understand what BigInt is. Big integer data type is intended for use when integer values might exceed the range that is supported by the int data type. The range is -2^63 (-9,223,372,036,854,775,808) to 2^63-1 (9,223,372,036,854,775,807) with storage size of 8 bytes. Generally this is used in SQL.
In Go, you can use a more precise integer types. Available built-in types int8, int16, int32, and int64 (and their unsigned counterparts) are best suited for data. The counterparts for big integer in Go is int64. With range -9,223,372,036,854,775,808 through 9,223,372,036,854,775,807, and storage size of 8 bytes.
Using mongo-go-driver,you can just use int64 which will be converted to bson RawValue.Int64. For example:
type MyStruct struct {
Number int64
}
collection := db.Collection("tests")
nb := MyStruct{Number: int64(42)}
r, _ := collection.InsertOne(context.TODO(), nb)
var result MyStruct
collection.FindOne(context.TODO(), bson.D{{"_id", r.InsertedID}}).Decode(&result)