Filtering post by category

Yes, see

https://cwiki.apache.org/confluence/display/solr/RequestHandlers+and+SearchComponents+in+SolrConfig

with

<lst name=”appends”>
<str name=”fq”>categories_str:my_category</str>
</lst>

Is there any hook where I can change query or do I have to edit your code? Cause I see

// Filter to change the solarium query
do_action( WpSolrFilters::WPSOLR_ACTION_SOLARIUM_QUERY,
	array(
		WpSolrFilters::WPSOLR_ACTION_SOLARIUM_QUERY__PARAM_SOLARIUM_QUERY => $solarium_query,
		WpSolrFilters::WPSOLR_ACTION_SOLARIUM_QUERY__PARAM_SEARCH_TERMS   => $wpsolr_query->get_wpsolr_query(),
		WpSolrFilters::WPSOLR_ACTION_SOLARIUM_QUERY__PARAM_SEARCH_USER    => wp_get_current_user(),
	)
);

but it is an action not filter, which as far as I know is not supposed to change anything.

If you feel comfortable with Solarium library, you can use this action indeed to add your own categories filter query.

Look for

add_action(WpSolrFilters::WPSOLR_ACTION_SOLARIUM_QUERY

in the code, and you’ll get some examples.