How should I apply this Eloquent relationship?

On your Post model:

public function user() 
{
    return $this->belongsTo('App\User');
}

public function category()
{
    return $this->belongsTo('App\Category');
}

Then in both your User and Category model you would have this function:

public function posts()
{
    return $this->hasMany('App\Post');
}

You have a one-to-many relationship between Users and Posts, as well as Categories and Posts. In the future if you want a post to be able to have multiple users or categories associated with it then you would use a many-to-many relationship.

http://laravel.com/docs/5.0/eloquent

Hey! thanks for the response. Actually thats what I did but I have not idea how to save a post this way (attaching category and user to a post)

Ah I see, I misread the question. All of that information is in the docs that I linked you, they're very straightforward.

You can also check out this video from Jeffrey, it's free in case you don't have a subscription yet. https://laracasts.com/series/laravel-5-fundamentals/episodes/8