codeigniter下拉列表不起作用

在下面的代码点火器代码中，我放置了控制器、模型和视图。在我的登录中，我在下拉列表中创建了用户名、密码和大学，以验证用户。但我在collecte_name下拉列表中出错消息：未定义的变量：validate，消息：为foreach（）提供的参数无效。请帮助我解决问题。控制器

function college()
{
    $this->load->helper('url');
    $data = array();
    $this->load->model("membership_model");
    $data['validate']  = $this->Membership_model->validate();

    $this->load->view('login_form');

}
function validate_credentials()
    {       
        $this->load->model('membership_model');
        $query = $this->membership_model->validate();
        if($query) // if the user's credentials validated...
        {
            $data = array(
                'username' => $this->input->post('username'),
                'is_logged_in' => true
            );
            if($query->num_rows()>0){
             $status = $query->row()->account_status;}
            else {
             $status = ''; }
             //Account active
            if($status == 'active')
            {
               $this->session->set_userdata($data);
               redirect('site1/members_area');
            }
            else  if ($status == 'inactive')//Account In active
            {  $this->inactive();
              }
              else // incorrect username or password
        {
            $this->invalid();
        }
        }
    }   

型号：

function validate()
    {
        $this->db->where('username', $this->input->post('username'));
        $this->db->where('password', md5($this->input->post('password')));
        $this->db->where('college_name', $this->input->post('college_name'));
          $query = $this->db->query("SELECT college_name FROM membership ");
        $query = $this->db->get('membership');
        return $query->result();
    }

视图：

<?php 
    echo form_open('login/validate_credentials');
    echo form_input('username', 'Username');
    echo form_password('password', 'Password');
    $array = array();
           foreach($validate as $row ){
    $array[] = $row->college_name;
}
            echo form_dropdown('validate',  $array);

    echo form_submit('submit', 'Login');
    echo anchor('login/signup', 'Create Account');
    echo form_close();
    ?>