[5.3] Improve the "with default" option in the has one relation.

[sileence]

There are 3 commits here:

1. First add test for the closure option.

2. Add option to pass an array of attributes for the default model, i.e.:

$this->hasOne(Profile::class)->withDefault(['avatar' => 'default.jpg']);

3. Make sure the closure is associated to the parent/related model. I'm allowing the possibility that the closure could return null.

[taylorotwell]

You would probably want to forceFill these.

[sileence]

@taylorotwell done.

[JosephSilber]

1. Why would the callback want to return null? That's a little weird.
2. Wouldn't it be better to pass a new linked-up model to the callback?

Maybe something like this:

protected function getDefaultFor(Model $model)
{
    if (! $this->withDefault) {
        return;
    }

    $instance = $this->related->newInstance()->setAttribute(
        $this->getPlainForeignKey(), $model->getAttribute($this->localKey)
    );

    if (is_callable($this->withDefault)) {
        return call_user_func($this->withDefault, $instance);
    }

    if (is_array($this->withDefault)) {
        $instance->forceFill($this->withDefault);
    }

    return $instance;
}

[sileence]

1. I don't know, I just wanted to leave that possibility.
2. I really like how you refactored the code 👍

[taylorotwell]

So are you going to refactor the code to Joseph's suggestion?

[sileence]

@taylorotwell done. If nothing is returned from the closure, the default instance will be returned automatically.

Thank you @JosephSilber! The code looks better now.