doctrine.php

<?php

class Repository 
{
    private $collection;
    public function __construct(Collection $collection) 
    {
        $this->collection = $collection;
    }
    
    public function findOneById($id)
    {
        $criteria = Criteria::create();
        $exp = \Doctrine\Common\Collections\Criteria::expr();
        $exp->eq('id', $id);
        $criteria->where($exp);
        
        return $collection->matching($criteria)->current();
    }
    
    public function findAll()
    {
        return $this->collection;
    }
    //other methods for save, findByCriteria etc?
}

$dbCollection = new \Doctrine\ORM\LazyCriteriaCollection(
    $em->getUnitOfWork()->getEntityPersister(Entity::class),
    new \Doctrine\Common\Collections\Criteria()
);

$dbRepository = new Repository($dbCollection);

$inMemoryCollection = new ArrayCollection();

$inMemoryRepository = new Repository($inMemoryCollection);