请问当有外键约束时只有这一种创建资源的方法吗

$request->user() 为什么这段可以这么写？具体实现代码在哪里？ 答：在Request中，会有

    /**
     * Set the user resolver callback.
     *
     * @param  \Closure  $callback
     * @return $this
     */
    public function setUserResolver(Closure $callback)
    {
        $this->userResolver = $callback;

        return $this;
    }

通过搜索调用者发现

    /**
     * Handle the re-binding of the request binding.
     *
     * @return void
     */
    protected function registerRequestRebindHandler()
    {
        $this->app->rebinding('request', function ($app, $request) {
            $request->setUserResolver(function ($guard = null) use ($app) {
                return call_user_func($app['auth']->userResolver(), $guard);
            });
        });
    }

这个$app['auth]实际上就是

    /**
     * Register the authenticator services.
     *
     * @return void
     */
    protected function registerAuthenticator()
    {
        $this->app->singleton('auth', function ($app) {
            // Once the authentication service has actually been requested by the developer
            // we will set a variable in the application indicating such. This helps us
            // know that we need to set any queued cookies in the after event later.
            $app['auth.loaded'] = true;

            return new AuthManager($app);
        });

注册的AuthManager.通过调查发现

        $this->userResolver = function ($guard = null) {
            return $this->guard($guard)->user();
        };

所以 $request->user() 实际上调用的是 'auth()->guard($guard)->user()'

user()->addresses() 为什么不是获取当前列表？它返回的是什么类？有其他方法可以获得表之间的关系吗？ addresses()->create() 使用 UserAddress::create() 无效，为什么这种方法可以。 $request->only() 这个在 UserAddress 中不是规定了 $fillable 吗 ，为什么还要 only() 答：通过查看user得知

    public function addresses()
    {
        return $this->hasMany(UserAddress::class);
    }

返回一个 Illuminate\Database\Eloquent\Relations 这个relation基类中会注入相关类的builder

    /**
     * The Eloquent query builder instance.
     *
     * @var \Illuminate\Database\Eloquent\Builder
     */
    protected $query;

    /**
     * Create a new relation instance.
     *
     * @param  \Illuminate\Database\Eloquent\Builder  $query
     * @param  \Illuminate\Database\Eloquent\Model  $parent
     * @return void
     */
    public function __construct(Builder $query, Model $parent)
    {
        $this->query = $query;
        $this->parent = $parent;
        $this->related = $query->getModel();

        $this->addConstraints();
    }

当create()时

    /**
     * Save a new model and return the instance.
     *
     * @param  array  $attributes
     * @return \Illuminate\Database\Eloquent\Model|$this
     */
    public function create(array $attributes = [])
    {
        return tap($this->newModelInstance($attributes), function ($instance) {
            $instance->save();
        });
    }

    /**
     * Create a new instance of the given model.
     *
     * @param  array  $attributes
     * @param  bool  $exists
     * @return static
     */
    public function newInstance($attributes = [], $exists = false)
    {
        // This method just provides a convenient way for us to generate fresh model
        // instances of this current model. It is particularly useful during the
        // hydration of new objects via the Eloquent query builder instances.
        $model = new static((array) $attributes);

        $model->exists = $exists;

        $model->setConnection(
            $this->getConnectionName()
        );

        $model->setTable($this->getTable());

        $model->mergeCasts($this->casts);

        return $model;
    }

可以看出返回的是一个新的模型并->save()

当new model的时候

    /**
     * Create a new Eloquent model instance.
     *
     * @param  array  $attributes
     * @return void
     */
    public function __construct(array $attributes = [])
    {
        $this->bootIfNotBooted();

        $this->initializeTraits();

        $this->syncOriginal();

        $this->fill($attributes);
    }

    /**
     * Fill the model with an array of attributes.
     *
     * @param  array  $attributes
     * @return $this
     *
     * @throws \Illuminate\Database\Eloquent\MassAssignmentException
     */
    public function fill(array $attributes)
    {
        $totallyGuarded = $this->totallyGuarded();

        foreach ($this->fillableFromArray($attributes) as $key => $value) {
            $key = $this->removeTableFromKey($key);

            // The developers may choose to place some attributes in the "fillable" array
            // which means only those attributes may be set through mass assignment to
            // the model, and all others will just get ignored for security reasons.
            if ($this->isFillable($key)) {
                $this->setAttribute($key, $value);
            } elseif ($totallyGuarded) {
                throw new MassAssignmentException(sprintf(
                    'Add [%s] to fillable property to allow mass assignment on [%s].',
                    $key, get_class($this)
                ));
            }
        }

        return $this;
    }

    /**
     * Get the fillable attributes of a given array.
     *
     * @param  array  $attributes
     * @return array
     */
    protected function fillableFromArray(array $attributes)
    {
        if (count($this->getFillable()) > 0 && ! static::$unguarded) {
            return array_intersect_key($attributes, array_flip($this->getFillable()));
        }

        return $attributes;
    }

所以$request->only()没有必要，因为model的$fillable会自动过滤