Go でカスタム BSON マーシャリングを実装するにはどうすればよいですか?

package yourpackage

import (
    "context"
    "errors"

    "github.com/globalsign/mgo/bson"
    "github.com/shopspring/decimal"
)

type Currency struct {
    Value        decimal.Decimal
    CurrencyCode string
}

func (c Currency) GetBSON() (interface{}, error) {
    if c.CurrencyCode == "" {
        return nil, errors.New("CurrencyCode cannot be empty")
    }

    return bson.M{
        "value":        c.Value.String(),
        "currencyCode": c.CurrencyCode,
    }, nil
}

func (c *Currency) SetBSON(raw bson.Raw) error {
    var m bson.M

    if err := raw.Unmarshal(&m); err != nil {
        return err
    }

    val, ok := m["value"]
    if !ok {
        return errors.New("missing 'value' field in BSON data")
    }

    c.Value, _ = decimal.NewFromString(val.(string))

    code, ok := m["currencyCode"]
    if !ok {
        return errors.New("missing 'currencyCode' field in BSON data")
    }

    c.CurrencyCode, _ = code.(string)

    return nil
}

session := mgo.Dial("mongodb://localhost:27017")
defer session.Close()

session.SetCollectionValidator("products",{
    "validator":{
        "$jsonSchema":{
            "bsonType":"object",
            "required":["name","code","price"],
            "properties":{
                "name":{"bsonType":"string"},
                "code":{"bsonType":"string"},
                "price":{
                    "bsonType":"object",
                    "required":["value", "currencyCode"],
                    "properties":{
                        "value":{"bsonType":"string", "pattern":"^[0-9,]+(.[0-9,]+)?$"},
                        "currencyCode":{"bsonType":"string", "enum":["USD", "EUR", "GBP"]},
                    },
                },
            }
        }
    }
})

collection := session.DB("products").C("products")