Multiple extension extend one extension in TYPO3 10 - typo3

Question

Szenario:
I have two extensions, which extend ext:news with some specific fields. Up to TYPO3 9 I had to configure the dependency to the news extension with the following TypoScript configuration:
config.tx_extbase {
persistence {
classes {
GeorgRinger\News\Domain\Model\News {
subclasses {
GeorgRinger\News\Domain\Model\News = Vendor\Extension\Domain\Model\News
}
}
Vendor\Extension\Domain\Model\News {
mapping {
tableName = tx_news_domain_model_news
}
}
}
}
}
The model Vendor\Extension\Domain\Model\News extends the model of the "base" extension:
class News extends \GeorgRinger\News\Domain\Model\News
In TYPO3 10 the TypoScript configuration was replaced with the following configuration in Configuration/Extbase/Persistence/Classes.php (Breaking: #87623):
\Vendor\Extension\Domain\Model\News::class => [
'tableName' => 'tx_news_domain_model_news',
'recordType' => 0,
],
This works as long, as you have just one extension which extends the news extension. If you have a second extension and enable the TYPO3 cache you will get an error, that the fields which are added in the first extension are not available in the templates of the news extension. The strange part is, that this problem only occurs, when enabling the cache!
So my question is:
What is the right way to add some fields to an existing extension in TYPO3 10?

Answer 1

As the changelog says, in TYPO3 versions below 10, your overriding configuration was located in typoscript. Since 10 LTS, it is located in a PHP class. As a consequence, the mechanism which controls loading order is no more the one for typoscript but the one for PHP.
Authoritative for the PHP loading order is the constraints section in the ext_emconf.php files of your extensions. They are parsed at first run and cached afterwards. The composer.json files do not control loading order - currently they only serve the purpose of resolving dependencies in composer itself.
In contrast to composer.json, the ext_emconf.php allows you to specify not only hard but also "soft" dependencies, which you can set using the suggests keyword. This way, you can specify loading order between extensions without them having to be installed in any case. This allows you to install each extension independently from the other, while you can still specify correct loading order in case they are both installed.
So, in your case, the second extension needs to have a soft "suggests" dependency to the first one:
constraints' => [
'depends' => [
],
'conflicts' => [
],
'suggests' => [
'news' => ''
'first_news_extension' => ''
],
]
See the complete explanation of constraints in ext_emconf here.