Handling Custom BSON Marshaling

Question

I have a number of structs that require custom marshalling. When I was testing I was using JSON and the standard JSON marshaller. As it doesn't marshal unexported fields, I needed to write a custom MarshalJSON function, which worked perfectly. When I called json.Marshal on the parent struct containing the ones that needed custom marshalling as fields, it worked fine.

Now I need to marshal everything to BSON for some MongoDB work, and I can't find any documentation about how to write custom BSON marshalling. Can anyone tell me how to do the equivalent for BSON/mgo for what I've demonstrated below?

currency.go (the important parts)

type Currency struct {
    value        decimal.Decimal //The actual value of the currency.
    currencyCode string          //The ISO currency code.
}

/*
MarshalJSON implements json.Marshaller.
*/
func (c Currency) MarshalJSON() ([]byte, error) {
    f, _ := c.Value().Float64()
    return json.Marshal(struct {
        Value        float64 `json:"value" bson:"value"`
        CurrencyCode string  `json:"currencyCode" bson:"currencyCode"`
    }{
        Value:        f,
        CurrencyCode: c.CurrencyCode(),
    })
}

/*
UnmarshalJSON implements json.Unmarshaller.
*/
func (c *Currency) UnmarshalJSON(b []byte) error {

    decoded := new(struct {
        Value        float64 `json:"value" bson:"value"`
        CurrencyCode string  `json:"currencyCode" bson:"currencyCode"`
    })

    jsonErr := json.Unmarshal(b, decoded)

    if jsonErr == nil {
        c.value = decimal.NewFromFloat(decoded.Value)
        c.currencyCode = decoded.CurrencyCode
        return nil
    } else {
        return jsonErr
    }
}

product.go (again, just the relevant parts)

type Product struct {
    Name  string
    Code  string
    Price currency.Currency
}

When I call json.Marshal(p) where p is a Product, it produces the output I want without the need for the pattern (not sure of the name) where you create a struct which is just a clone with all exported fields.

In my opinion using the inline method I've used greatly simplifies the API and stops you having extra structs that clutter things up.

Answer 1

Custom bson Marshalling/Unmarshalling works nearly the same way, you have to implement the Getter and Setter interfaces respectively

Something like this should work :

type Currency struct {
    value        decimal.Decimal //The actual value of the currency.
    currencyCode string          //The ISO currency code.
}

// GetBSON implements bson.Getter.
func (c Currency) GetBSON() (interface{}, error) {
    f := c.Value().Float64()
    return struct {
        Value        float64 `json:"value" bson:"value"`
        CurrencyCode string  `json:"currencyCode" bson:"currencyCode"`
    }{
        Value:        f,
        CurrencyCode: c.currencyCode,
    }, nil
}

// SetBSON implements bson.Setter.
func (c *Currency) SetBSON(raw bson.Raw) error {

    decoded := new(struct {
        Value        float64 `json:"value" bson:"value"`
        CurrencyCode string  `json:"currencyCode" bson:"currencyCode"`
    })

    bsonErr := raw.Unmarshal(decoded)

    if bsonErr == nil {
        c.value = decimal.NewFromFloat(decoded.Value)
        c.currencyCode = decoded.CurrencyCode
        return nil
    } else {
        return bsonErr
    }
}