Relationship not working as expected

[preliot]

When i call the endpoint /mail-addresses or /mail-addresses/<ID>?include=mail-domain no relationship attribute with 'mail-domain' is shown. But when i call /mail-addresses/<ID>/mail-domain i get the expected mail-domain resource! So what am i doing wrong?

In case you are wondering, /mail-domains does show the relationship attribute 'mail-addresses'.

I suspect someting is going on with pluralize or dash, but i cound't find the issue.

JSON API route

            $server->resource('mail-addresses', '\\' . MailAddressController::class)
                ->relationships(function ($relations): void {
                    $relations->hasOne('mail-domain')->readOnly();
                });

MailAddressSchema

class MailAddressSchema extends Schema
{
    /**
     * The model the schema corresponds to.
     *
     * @var string
     */
    public static string $model = MailAddress::class;

    /**
     * Get the resource fields.
     *
     * @return array
     */
    public function fields(): array
    {
        return [
            ID::make()->uuid(),           
            //...
            BelongsTo::make('mail-domain', 'mailDomain')
                ->type('mail-domains')
                ->readonly(),
        ];
    }

MailAddress Model

class MailAddress extends BaseModel
{
    use SoftDeletes;

   //...

    /**
     * Get mail domain it belongs to.
     */
    public function mailDomain(): BelongsTo
    {
        return $this->belongsTo(MailDomain::class, 'mail_domain_id');
    }

MailAddressPolicy

class MailAddressPolicy extends BasePolicy
{
    public function viewMailDomain(User $user, Model $model): bool
    {
        return $this->view($user, $model);
    }
}

[BenWalters]

Looks okay to me... What's in your MailAddressController?

[preliot]

Found it! It was an empty relationships method in a custom MailAddressResource class:

 /**
     * Get the resource's relationships.
     *
     * @param Request|null $request
     * @return iterable
     */
    public function relationships($request): iterable
    {
        return [
            // @TODO
        ];
    }

Somehow i've missed this :-(