gqlgen/Lobby

@ethanrubio

You get it from the ID. According to the spec, the id is a global identifier, and using the id would guarantee that the same object always gets returned. So matching it on __typename would probably be breaking the relay spec.

I solved it like this:

func (r *queryResolver) Node(ctx context.Context, globalId string) (Node, error) {
    loaders := tenants.LoadersFromContext(ctx)
    nodeType, id, err := fromGlobalId(globalId)
    if err != nil {
        return nil, err
    }
    switch nodeType {
    case "User":
        return loaders.User().Load(id)
    case "Organization":
        return loaders.Organization().Load(id)
    case "Group":
        return loaders.Group().Load(id)
    default:
        return nil, fmt.Errorf("type not supported: %s", nodeType)
    }
}

var globalIdSeparator = ":"

func toGlobalId(typeName string, id primitive.ObjectID) string {
    return base64.StdEncoding.EncodeToString([]byte(typeName + globalIdSeparator + id.Hex()))
}

func fromGlobalId(globalId string) (typeName string, id primitive.ObjectID, err error) {
    decoded, err := base64.StdEncoding.DecodeString(globalId)
    if err != nil {
        return "", primitive.NilObjectID, nil
    }
    ids := strings.Split(string(decoded), globalIdSeparator)
    if len(ids) != 2 {
        return "", primitive.NilObjectID, fmt.Errorf("invalid global id format")
    }
    typeName = ids[0]
    id, err = primitive.ObjectIDFromHex(ids[1])
    return
}

Of course, that requires a resolver for each ID in all models.
It would probably be nice if it was possible to create a generic ID resolver/marshaller that gets the entire model and not just the field value (to be able to get the model type), not sure if that's possible.