MyProject/app/models/model_main.php

<?php

class Model_main extends Model
{
    public function get_posts(){
        $q = "SELECT * FROM `posts` ORDER BY `updated_at` DESC";
        return $this->db->query($q);
    }

    public function create_post($caption, $post){
        $q = "INSERT INTO `posts` SET `caption` = " . $this->db->escape($caption) .
            ", `post` = " . $this->db->escape($post) .
            ", `user_id` = " . $this->db->escape($_SESSION['user']['id']);
        $this->db->query($q);

    }

}