Passing array in many to many relationship to the controller- laravel - php

Question

I have a many to many relationship between Category and posts tables like this:
Category class:
class Category extends Model {
public function posts()
{
//return $this->hasMany('App\Posts','category_id');
return $this->belongsToMany('App\Posts', 'categories_post', 'category_id', 'post_id')
->withTimestamps();
}
}
Posts class:
class Posts extends Model {
public function category()
{
//return $this->belongsTo('App\Category','category_id');
return $this->belongsToMany('App\Category', 'categories_post', 'post_id', 'category_id')
->withTimestamps();
}
}
When I need to access only one category's posts I do this in my controller:
public function cat($category_id)
{
$posts= $category_id->posts()->paginate(7);
return view('cat')->with('posts',$posts);
}
Edit
to make this work, I added this in "RouteServiceProvider.php" file:
public function boot(Router $router)
{
parent::boot($router);
$router->model('categories','App\Category');
}
and this works perfectly. The problem is, I have another controller which should get the posts for multiple categories :
public function index()
{
$title = 'news';
$categories= [1, 2, 10, 11];
// Here is the problem:
$posts= $categories->posts()->paginate(7);
return view('news')->with('posts',$posts)->with('title',$title);
}
This gives me this error: Call to a member function posts() on a non-object
I know there is something wrong with calling the array, but I don't know how to solve it. Can any one help me please :)
The solution
after what Thomas Van der Veen said in his answer , I came up with this controller which is working perfectly:
public function index()
{
$title = 'news';
$category_ids = [1, 2, 10, 11];
$posts = Posts::whereHas('category', function($query) use ($category_ids) {
$query->whereIn('id', $category_ids);
})->get();
return view('news')->with('posts',$posts)->with('title',$title);
}

Answer 1

You could do something like:
$category_ids = [1, 2, 10, 11];
$posts = Post::whereHas('categories', function($query) use ($category_ids) {
$query->whereIn('id', $category_ids);
});
See this.