Codeigniter - Using Multiple Databases

I got it to work. First I edited my database.php configuration file to include the new database. Then, in my model, I declared a class variable $current_db, and I wrote the following two functions:

function getDB($db){
    if($db ==1){
        $this->current_db  = $this->load->database('default', TRUE);
    } elseif($db ==2) {
        $this->current_db  = $this->load->database('local', TRUE);
    }
}

function dyno_query($table, $id){
    $query = $this->current_db->get_where($table, array('id' => $id));
    return $query->result_array();
}

Then in my controller, I ran the following:

$arr = array();
$this->My_Model->getDB(1);
$arr['default'] = $this->My_Model->dyno_query('default_table', 2);
$this->My_Model->getDB(2);
$arr['local'] = $this->My_Model->dyno_query('local_table', 74);
var_dump($arr);

The result was an array with data from both databases. Maybe the key to it all is defining a class variable with a name other than $db.

EDIT:

To keep the current database loaded on each page load, you'll need to use sessions. In the controller function that handles the dropdown data, you could enter something similar to the following:

$db = $this->input->post('select')
$this->My_Model->getDB($db);
$this->session->set_userdata($db);

Within any controller that will use the database you'll want to add code to load the current database:

function __construct(){
    parent::__construct();
    $this->load->model('My_Model');
    $db = $this->session->userdata('db');
    $this->My_Model->getDB($db);
}

EDIT:

If you are going to need to access the same database from various models, I suggest using a library by mddd at EllisLab. Just create a PHP file named Db_manager.php with the folowing code and drop it into your application/libraries directory:

class Db_manager
{
    var $connections = array();
    var $CI;

    function __construct()
    {
        $this->CI =& get_instance();
    }

    function get_connection($db_name)
    {
        // connection exists? return it
        if (isset($this->connections[$db_name])) 
        {
            return $this->connections[$db_name];
       }
       else
       {
            // create connection. return it.
            $this->connections[$db_name] = $this->CI->load->database($db_name, true);
            return $this->connections[$db_name];
        }
    }
}

In the constructor of each model that will use the database add the following:

var $current_db;

function __construct(){
    parent::__construct();
    $this->load->library('Db_manager');
    $db = $this->session->userdata('db');
    if ($db == 1){
        $this->current_db = $this->db_manager->get_connection('default');
    } else {
        $this->current_db = $this->db_manager->get_connection('alternate');
    }
}

From the docs ( https://www.codeigniter.com/user_guide/database/connecting.html ) :

The first parameter of this function can optionally be used to specify a particular database group from your config file, or you can even submit connection values for a database that is not specified in your config file.

So you would do something like this, replacing the values with values from the master database:

$config['hostname'] = "localhost";
$config['username'] = "myusername";
$config['password'] = "mypassword";
$config['database'] = "mydatabase";
$config['dbdriver'] = "mysql";
$config['dbprefix'] = "";
$config['pconnect'] = FALSE;
$config['db_debug'] = TRUE;
$config['cache_on'] = FALSE;
$config['cachedir'] = "";
$config['char_set'] = "utf8";
$config['dbcollat'] = "utf8_general_ci";

$this->load->database($config);

If you need to maintain a connection to the master database and the customer database, then change the last line to:

$customer_db = $this->load->database($config, TRUE);

// to use the master database:
$this->db->query("SELECT * FROM my_table");

// to then use the customer database:
$customer_db->query("SELECT * FROM whatever");