If I were in your position, I'd start by slowly decoupling your application from codeigniter. Isolate the "business logic" type stuff from controllers and move it into their own classes that you instantiate inside the controller. Then, you need to do the same thing for that code, except with the database interactions. Once that's done, you can start to properly test it in isolation.
You have to essentially repeat that process for business logic of your application. While you're at it, I would strongly encourage you consider use entities and value objects that aren't tightly-coupled to the names of your database fields. For example:
interface ProductEntity
{
public function getId();
public function getDetails();
public function getPrice();
public function getStockLevels();
}
Now, make the interface work properly with your old CI model:
class ProductModel extends CI_Model implements ProductEntity
{
var $poorlyNamedIdField;
var $poorlyNamedPriceField;
var $equallyWeirdTitle;
var $alsoWeirdDescriptionField;
/**
* If you change anything about this, the system will break everywhere at once
* @author Developer Who Hates You
* @var Array|ProbablyAnArray|Cthulu
*/
var $newUpdated_correct_stockLevels;
public function getId()
{
return $this->poorlyNamedIdField;
}
public function getDetails()
{
return new ProductDetails($this->equallyWeirdTitle, $this->alsoWeirdDescriptionField);
}
public function getStockLevels()
{
return new ProductStockLevel(
$this->newUpdated_correct_stockLevels['total'],
$this->newUpdated_correct_stockLevels['in_buffer'],
$this->newUpdated_correct_stockLevels['available']
);
}
}
And your new Eloquent product model:
class EloquentProduct extends Eloquent implements ProductEntity
{
protected $table = 'products';
public function getId()
{
return $this->id;
}
public function getDetails()
{
return new ProductDetails($this->title, $this->description);
}
public function getStockLevels()
{
return new ProductStockLevel(
$this->stock->total,
$this->stock->shipping_buffer,
$this->stock->available
);
}
}
Obviously, a real implementation will be harder, but you get the idea. The interface will let you refactor with your old implementation, but let yo use eloquent in your new one without rewriting everything that interacts with the models.
For stuff like services, you can do something similar by extracting them into a composer package. In that package, include a class that makes the service work with your current CI implementation, but also create a service provider designed to work with Laravel.
Basically, the whole process is slowly pulling your business logic away from the presentation/persistence/http layers of CodeIgniter. It'll be a long process, but easier than a full rewrite.
