Laravel 5 Querying with relations causes “Call to a member function addEagerConstraints() on null” error

You are missing return statements in the methods that define relations. They need to return relation definition.

Replace

public function roles()
{
    $this->belongsToMany('\App\Role', 'fk_user_role', 'user_id', 'role_id');
}

With

public function roles()
{
    return $this->belongsToMany('\App\Role', 'role_user', 'user_id', 'role_id');
}