Calculate value with data from another model

Hi, this is code from my seminar model:

PHP Code:

// Seminar Model

public function find_by_id($id) {
 $this->load->model('Seminar_date_model', 'seminar_date');
 $query = $this->db->where('seminar_id', $id)->get('seminar_dates');
 $seminar_dates= $query->custom_result_object('Seminar_date_model');
 
 $query = $this->db->where('id', $id)->get('seminars');
 $result = $query->custom_row_object(0, 'Seminar_model');
 $result->seminar_dates = $seminar_dates;
 return $result;
}

public function get_seminar_dates() {
    // if i call $this->seminar_dates->price() there are no seminar data
    return $this->seminar_dates;
} 


In the seminar_dates model i want to calculate a price and need data from the seminar model. How can i solve this?
Getting the seminar_date model with an seminar is easy, because then i have access to the seminar property in the seminar_date model by doing this:

PHP Code:

// Seminar_date Model

public function find_by_id($id) {
    $query = $this->db->where('id', $id)->get('seminar_dates');
    $result = $query->custom_row_object(0, 'Seminar_date_model');
 
    $this->load->model('Seminar_model', 'seminar');
    $query = $this->db->where('id', $result->seminar_id)->get('seminars');
    $seminar = $query->custom_row_object('Seminar_model');
    $result->seminar = $seminar;

    return $result;
}

public function price() {
    // seminar data available 
    return $this->seminar->price - $this->coupon;
}