HOWTO: Extend TYPO3/extbase Model with new fields

gistfile1.txt

1. Create ext_tables.sql and add new fields as usual

2. Create Configuration/TCA/Overrides/tablename.php and extend TCA therein (before: ext_tables.php)

3. Add the following lines to your ext_localconf.php. Where the first argument of "registerImplementation" is the model class you would like to extend and the second argument is your own implementation.
$extbaseObjectContainer = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Extbase\\Object\\Container\\Container');
$extbaseObjectContainer->registerImplementation('Tx_News_Domain_Model_NewsDefault', 'thinkopen_at\T3ff\Domain\Model\NewsT3ff');

4. Create your own implementation of the mode class and add the new properties + getters/setters for them

5. Add the following TypoScript:
--------------------------------------------
config.tx_extbase.persistence.classes {
        # Base model
        Tx_News_Domain_Model_News {
                subclasses {
                        # Take care to use an unique number
                        # Specify your own model class here
                        3 = thinkopen_at\T3ff\Domain\Model\NewsT3ff
                }
        }

        # Specify a tableName (column) mapping here
        thinkopen_at\T3ff\Domain\Model\NewsT3ff {
                mapping {
                        recordType = 0
                        tableName = tx_news_domain_model_news
                }
        }
}

6. Clear all caches