Error: Uncaught Error: Call to a Member Function prepare() on null

Question

I am trying to list a database table however I am not succeeding and getting the error "Fatal error: Uncaught Error: Call to a Member Function prepare() on null in C: xampp htdocs service_desk_pi classes Model.php:28 Stack trace: #0 C: xampp htdocs service_desk_pi equipamento.php(7): Model->listar() #1 {main} thrown in C:xampp htdocs service_desk_pi classes Model.php on line 28"

Model.php

<?php
require 'Conexao.php';
class Model
{

    protected $tabela;
    protected $class;
    protected $db;

    public function __construct()
    {
        $conexao = new Conexao();
        $this->db = $conexao->conectar();
    }

    public function inserir($values)
    {
        $sql = "INSERT INTO {$this->tabela} VALUES ($values)";
        $stmt = $this->db->prepare($sql);
        $stmt->execute();
        return $this->db->lastInsertId();
    }

    public function listar()
    {
        $sql = "SELECT * FROM {$this->tabela}";
        $stmt = $this->db->prepare($sql);
        $stmt->setFetchMode(PDO::FETCH_CLASS, $this->class);
        $stmt->execute();
        return $stmt->fetchAll();
    }

    public function get($id)
    {
        $sql = "SELECT * FROM {$this->tabela} WHERE id = {$id}";
        $stmt = $this->db->prepare($sql);
        $stmt->setFetchMode(PDO::FETCH_CLASS, $this->class);
        $stmt->execute();
        return $stmt->fetch();
    }

    public function alterar($id, $values)
    {
        $sql = "UPDATE {$this->tabela} SET {$values} WHERE id = {$id}";
        $stmt = $this->db->prepare($sql);
        $stmt->execute();
    }
    public function deletar($id)
    {
        $sql = "DELETE FROM {$this->tabela} WHERE id = {$id}";
        $stmt = $this->db->prepare($sql);
        $stmt->execute();
    }
} 

Connexion.php

<?php

class Conexao
{
    private $host;
    private $user;
    private $password;
    private $conn;

    public function __construct() {
        $this->host = 'mysql:host=localhost;dbname=teste';
        $this->user = 'root';
        $this->password = '';
    }

    public function conectar() {
        try {
            $conn = new PDO(
                            $this->host,
                            $this->user, 
                            $this->password
                        );

            return $conn;

        } catch (PDOException $e) {
            echo 'Erro na conexao. Erro reportado: ' . $e->getMessage();
            exit;
        }
    }
}

Equipmentdao.php

<?php
require 'Model.php';
class EquipamentoDAO extends Model
{   
    public function __construct() {
        $this->tabela = 'equipamento';
        $this->class = 'Equipamento';
    }
} 

index php.

<?php include 'layout/header.php'; 
?>
<?php 
require 'classes/Equipamento.php';
require 'classes/EquipamentoDAO.php';
$equipamentoDAO = new EquipamentoDAO();
$equipamentos = $equipamentoDAO->listar();


?>

<div class="row" style="margin-top:40px">
   <div class="col-10">
       <h2>Gerenciar equipamentos</h2>
   </div>
   <div class="col-2">
       <a href="form_equipamento.php" class="btn btn-success">Nova</a>
   </div>
</div>
<div class="row">
   <table class="table table-hover table-bordered table-striped">
       <thead>
           <tr>
               <th>#ID</th>
               <th>id_chamado</th>
               <th>Marca</th>
               <th>Modelo</th>
               <th>Categoria</th>
           </tr>
       </thead>
       <tbody>
           <?php foreach($equipamentos as $equipamento){ ?>
           <tr>
               <td><?= $equipamento->getId() ?></td>
               <td><?= $equipamento->getMarca() ?></td>
               <td>
                   <a href="form_equipamento.php?id=<?= $equipamento->getId() ?>">Editar</a> | 
                   <a href="controle_equipamento.php?acao=deletar&id=<?= $equipamento->getId() ?>" onclick="return confirm('Deseja realmente excluir?')">Excluir</a>
               </td>
           </tr>
           <?php } ?>
       </tbody>
   </table>
</div>

<?php include 'layout/footer.php'; ?> 

Answer 1

The problem is that as you declared a constructor in the Equipmentdao class, the constructor of the Model class will not run automatically.

If you add the code below in the constructor of the Equipmentclass your problem should be solved

parent::__construct();