When I run script I get error

Fatal error: Uncaught Error: Call to a member function prepare() on string in /Applications/MAMP/htdocs/sae/TT/public_html/includes/user.php:21 Stack trace: #0 /Applications/MAMP/htdocs/sae/TT/public_html/includes/user.php(40): User->emailExists('user1@gmail.com') #1 /Applications/MAMP/htdocs/sae/TT/public_html/includes/user.php(102): User->createUserAccount('userr', 'user1@gmail.com', '123456789', 'Admin') #2 {main} thrown in /Applications/MAMP/htdocs/sae/TT/public_html/includes/user.php on line 21

Please have a look in code and help me find out error.

        <?php
        ini_set("display_errors", 1); error_reporting(E_ALL);
        class User
        {
            private $con;

            function __construct()
            {
                include_once("../database/db.php");
                $db = new Database();
                $this->con = $db->connect();
                // if ($this->con) {
                //  echo "COnnected";
                // }

            }
        // Check user 
        private function emailExists($email)
        {
            $pre_stmt=$this->con->prepare("SELECT id FROM user WHERE email = ? ");
            $pre_stmt->bind_param("s", $email);
            $pre_stmt->execute() or die($this->con->error);
            $result = $pre_stmt->get_result();
            if ($result->num_rows>0) {
                return 1;

            } else
            {
                return 0;

            }

    }

    public function createUserAccount($username,$email,$pass_hash,$usertype)
    {
        if ($this->emailExists($email)) {
            return "EMAIL_ALREADY_EXISTS";

        }else{
            $pass_hash = password_hash($password,PASSWORD_BCRYPT,["cost"=>8]);
            $date = date("Y-m-d");
            $notes = "";
            $pre_stmt = $this->con->prepare("INSERT INTO `user`(`username`, `email`, `password`, `usertype`, `register_date`, `last_login`, `notes`) VALUES (?,?,?,?,?,?,?)");
            $pre_stmt->bind_param("sssssss", $username, $email, $pass_hash, $usertype, $date, $date, $notes);
            $result = $pre_stmt->execute() or die($this->con->error);
            if ($result) {
                return $this->con->insert_id;
            } else {
                return "SOME_ERROR";
            }

        }

    }
    public function userLogin($email,$password)
    {
        $pre_stmt = $this->con->prepare("SELECT id,username,password,last_login FROM user WHERE email =? ");
        $pre_stmt->bind_param("s",$email);
        $pre_stmt->execute() or die($this->con->error);
        $result = $pre_stmt->get_result();

        if ($result->num_rows <1) {
            return "NOT_REGISTERD";

        }else{
            $row = $result->fetch_assoc();
            if (password_verify($password,$row['password'])) {
                $_SESSION['userid'] = $row['id'];
                $_SESSION['username'] = $row['username'];
                $_SESSION['last_login'] = $row['last_login'];
                // updating user last_login

                $last_login = date("y-d-m h:m:s");
                $pre_stmt = $this->con->prepare("UPDATE user SET last_login = ? WHERE email = ? ");
                $pre_stmt->bind_pram("ss",$last_login,$email);
            $result = $pre_stmt->execute() or die($this->con->error);
            if ($result) {
                return 1;

            }else{
                return 0;
            }

        }else{
            return "PASSWORD_NOT_MATCHED";
        }

    }

    }
}
$user = new User();
echo $user->createUserAccount("userr", "user1@gmail.com", "123456789", "Admin");
echo $user->userLogin("user1@gmail.com","123456789");
?>

Include database (line 9) should be outside of object User because in line 11 you define reference to db connection which is lost after comlete user construct