Magento database model without an auto_increment primary key

When you create a model in Magento that reads and writes data $model->load() and $model->save() to a database table, by default, Magento expects the primary key of the table to be an auto_increment field called id.

If you want to use a primary key that is not an auto_increment field then you need to set the _isPkAutoIncrement flag to false.

For example:

class {Vendor}_{Extension}_Model_Resource_{Model} extends Mage_Core_Model_Resource_Db_Abstract 
{
    public function _construct()
    {    
        $this->_init('{model alias}/{table alias}', 'entity_id'); // Table resource, primary key
        $this->_isPkAutoIncrement = false; // The primary key is not an auto_increment field
    }
}

If the flag is true (the default), then the save() method will always do an update when you provide a value for the primary key and an insert otherwise. That’s just what you want when the primary key is an auto_increment. If the flag is false, then Magento checks for the existence of a row with that primary key value first. If it exists, it does an update and if it doesn’t exist then it does an insert.

evgv/mage_model_without_auto_increment_primary_key.markdown

Magento database model without an auto_increment primary key