spec.php

<?php
class CategorySpecification
{
  private $category;

  public function __construct($category)
  {
    $this->category = $category;
  }
  
  public function __invoke($qb)
  {
    $qb->andWhere(..);
  }
}

class ArticleQuery extends QueryObject
{
  private $specifications = [];
  
  public function addSpecification($spec)
  {
    $this->specifications[] = $spec;
  }
  
  protected function doCreateQuery($queryable)
  {
    $qb = ...;
    foreach ($this->specifications as $spec) {
      $spec($qb);
    } 
    
    return $qb;
  }
}