Fatal error: Call to a member function where() on a non-object in

I've been debugging this code but still not successful. Can anyone help me out please?

class Membership_model extends CI_Model {

function __construct()
{
    parent::__construct();
}

function validate()
{
    $this->db->where('username', $this->input->post('username'));
    $this->db->where('password', md5($this->input->post('password')));
    $query = $this->db->get('membership');

    if($query->num_rows == 1)
    {
        return true;
    }
}

function create_member()
{

    $new_member_insert_data = array(
        'first_name' => $this->input->post('first_name'),
        'last_name' => $this->input->post('last_name'),
        'email_address' => $this->input->post('email_address'),         
        'username' => $this->input->post('username'),
        'password' => md5($this->input->post('password'))                       
    );

    $insert = $this->db->insert('membership', $new_member_insert_data);
    return $insert;
}
}

I kept on getting an fatal error on the line

$this->db->where('username',$this->input->post('username'));

this is the controller/login.php

class Login extends CI_Controller {

function __construct()
{
    parent::__construct();
}

function index()
{
    $this->load->helper('url');
    $data['main_content'] = 'login_form';
    $this->load->view('includes/template', $data);      
}

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
        );
        $this->session->set_userdata($data);
        redirect('site/members_area');
    }
    else // incorrect username or password
    {
        $this->index();
    }
}   

function signup()
{
    $data['main_content'] = 'signup_form';
    $this->load->view('includes/template', $data);
}

function create_member()
{
    $this->load->library('form_validation');

    // field name, error message, validation rules
    $this->form_validation->set_rules('first_name', 'Name', 'trim|required');
    $this->form_validation->set_rules('last_name', 'Last Name', 'trim|required');
    $this->form_validation->set_rules('email_address', 'Email Address', 'trim|required|valid_email');
    $this->form_validation->set_rules('username', 'Username', 'trim|required|min_length[4]');
    $this->form_validation->set_rules('password', 'Password', 'trim|required|min_length[4]|max_length[32]');
    $this->form_validation->set_rules('password2', 'Password Confirmation', 'trim|required|matches[password]');


    if($this->form_validation->run() == FALSE)
    {
        $this->load->view('signup_form');
    }

    else
    {           
        $this->load->model('membership_model');

        if($query = $this->membership_开发者_开发百科model->create_member())
        {
            $data['main_content'] = 'signup_successful';
            $this->load->view('includes/template', $data);
        }
        else
        {
            $this->load->view('signup_form');           
        }
    }

}

function logout()
{
    $this->session->sess_destroy();
    $this->index();
}

The database probably is not being initialized properly before validate is called.

Looks like you're not connected to your DB. Make sure you are connecting to your database.

You can either connect automatically everytime your script runs or connect to the DB manually. Look at the CI guides for your connection options : http://codeigniter.com/user_guide/database/connecting.html

Could it be that the load->model( 'membership_model' ) should receive the name of a class, and that that name is case sensitive? You should probably check the return value of the API functions...