Need refactoring advice

The only other suggestion I could make would be to configure an autoloader (like Composer), create a model which doesn't extend CI_Model, and don't use CI's loader to load the model. Then you can pass the ID to the constructor.

The downside, of course, is that you have to make sure to load the database and you'll have to copy over the magic method from CI_Model (to access the CI instance via $this) if you want to limit the amount of code you'll need to change in your model.

The upside is that you could connect directly to the right database in the model's constructor, rather than switching databases.

Bonfire

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