Description by Tyler Harms:
Sort clauses content using \eZ\Publish\API\Repository\Values\Content\Query\SortClause\Location\Priority are ignored by Solr because priority_id doesn't exist.
Missing parameter solr_dsn on setting up Solr on 5.4
PR by Tyler Harms
Tested and approved by QA on 5.4 and master.
Pull-request by Tyler Harms: https://github.com/ezsystems/ezplatform-solr-search-engine/pull/28.
Merged to master@0e3a10b.