路由模型绑定不从id解析模型

I am building an application in Laravel 5.7. I have set up a route in my routes/api.php file which is bound to the user model. When I pass an existing user ID into the route it is being resolved to a blank user model.

I believe I have followed the laravel standard for this by type-hinting for a user model in the controller.

routes/api.php

Route::prefix('role-permissions')->group(function () {
    Route::get('by-user/{user}', 'RolePermissionsController@getByUserId')->name('api.role-permissions.by-user');
});

app/Http/Controllers/RolePermissionsController.php

public function getByUserId(User $user)
{
    dd($user);
    return $this->getAsResourceCollection($this->repository->getByUserId($user));
}

the dd($user); returns a blank User object

Expected: A filled user model with the correct id I can confirm a user with Id 1 DOES exist in my table

Actual:

User {#375
  #fillable: array:3 [
    0 => "name"
    1 => "email"
    2 => "password"
  ]
  #hidden: array:2 [
    0 => "password"
    1 => "remember_token"
  ]
  #connection: null
  #table: null
  #primaryKey: "id"
  #keyType: "int"
  +incrementing: true
  #with: []
  #withCount: []
  #perPage: 15
  +exists: false
  +wasRecentlyCreated: false
  #attributes: []
  #original: []
  #changes: []
  #casts: []
  #dates: []
  #dateFormat: null
  #appends: []
  #dispatchesEvents: []
  #observables: []
  #relations: []
  #touches: []
  +timestamps: true
  #visible: []
  #guarded: array:1 [
    0 => "*"
  ]
  #rememberTokenName: "remember_token"
  #accessToken: null
}

I had removed the api middleware route the mapApiRoutes method in the RouteServiceProvider

I don't think the $user should be object. It is the parameter passed in route, so the code should be :

public function getByUserId($user)
{
    dd($user);
    return $this->getAsResourceCollection($this->repository->getByUserId($user));
}