Support LIKE and NOT LIKE operators in database backend

NR for getting an idea if this could be supported.

Thanks for the suggestion!
Sure, if using those operators already works correctly when using custom code to override the filter plugins then it makes sense not to prevent this use case in the backend plugin’s code. Seems a very simple change for the moment.
See also ✨ Implement like operator for search_api_db Active , which discusses a more elaborate implementation that also offers the new operators in the UI.
However, this would still need some test coverage before it can be committed. Ideally, we’d already test this in combination with Views, when using Core’s built-in string plugin as you describe. But just making sure it can be used internally would already be fine, too, I think.

Also, as you already found out, testing issue forks still doesn’t work with this module, so please still use patches for now. Now that there’s finally been movement in the test bot issue queue we’ll hopefully have this resolved soon.
Attaching a patch with some small code style changes.

NW for the tests.

I have tried this patch but I have got one question, how do I alter the operator for the Fulltext search ?
Tried something like this:

  function mymodule_search_api_query_alter(\Drupal\search_api\Query\QueryInterface &$query) {
    foreach ($conditionGroups as &$conditionGroup) {
      foreach ($conditionGroup->getConditions() as &$condition) { 
        // no condition found for the fulltext filter.
      }
  }
}

but the condition for the Fulltext search filter is not there, did I miss something or is it something unusual ?

The fulltext search is actually not a “condition” in the Search API, but a separate part of the search query, the “keys”.
You can retrieve them via $query->getKeys() – they’ll usually be represented as an array structure, see the doc block of \Drupal\search_api\ParseMode\ParseModeInterface::parseInput() for details.

Fulltext search basically just has two operators, AND (“contains all of these words”) and OR (“contains any of these words”). To switch to OR, you can either do

    $query->getParseMode()->setConjunction('OR');

before the keywords are set on the query or

    $keys = &$query->getKeys();
    if (is_array($keys)) {
      $keys['#conjunction'] = 'OR';
    }

afterwards.

If $keys is a string, not an array, than the way to express an OR conjunction depends on the backend. (For the database backend, there is no way to do that with unparsed keywords, i.e., if $keys is a string. Use the “Terms” parse mode instead.)