Laravel 5 says: "Unknown column 'user_id' in 'field list'", but I have 'author_id'

I have user and post models have this relation:

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

// User
public function articles() {
    return $this->hasMany('App\BlogPost');
}

In database I have "blog_posts" table with "author_id" field, but Laravel returns this error:
"Column not found: 1054 Unknown column 'user_id' in 'field list'".
What can I do?!

via Chebli Mohamed