How to Ignore Nulls When Unmarshaling MongoDB Documents in Go?

Ignoring Nulls During MongoDB Document Unmarshaling

Mongo-Go-Driver provides several approaches to ignore nulls when unmarshalling MongoDB documents into Go structs.

1. Custom Decoders for Specific Types

• Create custom decoders for each type that can handle null values.
• Set the empty value (e.g., "" for strings, 0 for integers) when encountering null.
• Register the decoders within a registry and set it on the mongo.Client or other relevant objects.

2. "Type-Neutral" Null-Aware Decoders

• Create a generic decoder that handles null values for all target types.
• Set the decoded value to the type's zero value, effectively ignoring null values.
• Register the decoder for specific types or all types using a RegistryBuilder.

Code Sample:

// Nullaware decoder for all types, setting null values to zero values
type NullawareDecoder struct {
    DefDecoder bsoncodec.ValueDecoder
    ZeroValue  reflect.Value
}

// Decodes null values to zero values, otherwise delegates to the default decoder
func (nd *NullawareDecoder) DecodeValue(dctx bsoncodec.DecodeContext, vr bsonrw.ValueReader, val reflect.Value) error {
    if vr.Type() == bsontype.Null {
        if !val.CanSet() {
            return errors.New("value not settable")
        }
        if err := vr.ReadNull(); err != nil {
            return err
        }
        val.Set(nd.ZeroValue)
        return nil
    }
    return nd.DefDecoder.DecodeValue(dctx, vr, val)
}

// Register NullawareDecoder for desired types
rb := bson.NewRegistryBuilder()
rb.RegisterDecoder(reflect.TypeOf(""), &NullawareDecoder{bson.DefaultRegistry.LookupDecoder(reflect.TypeOf("")), reflect.ValueOf("")})
// ... (register for other types as needed)

// Use the registry within a ClientOptions instance
clientOpts := options.Client().
    ApplyURI("mongodb://localhost:27017/").
    SetRegistry(rb.Build())

// Initialize MongoDB client with customized registry to ignore nulls
client, err := mongo.Connect(ctx, clientOpts)

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