/**
* @Route("/search/city", name="search_city")
*
* @param Request $request
* @return \Symfony\Component\HttpFoundation\Response
*/
public function searchCityAction(Request $request)
{
$search = new City();

$form = $this->createForm(CityType::class, $search)
->add('submit', SubmitType::class, ["label" => "Szukaj"]);


$entityManager = $this->getDoctrine()->getManager();


if($request->isMethod(Request::METHOD_POST))
{
$form->handleRequest($request);

$selectedCity = ($_REQUEST['city']['city']);

$address = $entityManager->getRepository(Address::class)->findBy(["city" => $selectedCity]);
$searched = $entityManager->getRepository(Advert::class)->findBy(["address" => $address]);

return $this->render("Search/searched.html.twig", ["searched" => $searched]);
}


return $this->render("Search/index.html.twig", ["form" => $form->createView()]);
}

/**
* @Route("/search/city", name="search_city")
*
* @param Request $request
* @return \Symfony\Component\HttpFoundation\Response
*/
public function searchCityAction(Request $request)
{
$search = new City();

$form = $this->createForm(CityType::class, $search)
->add('submit', SubmitType::class, ["label" => "Szukaj"]);


$entityManager = $this->getDoctrine()->getManager();


if($request->isMethod(Request::METHOD_POST))
{
$form->handleRequest($request);

$selectedCity = ($_REQUEST['city']['city']);

$address = $entityManager->getRepository(Address::class)->findBy(["city" => $selectedCity]);
$addressIds = $address->map(function($single_address) {
return $single_address->id;
});
$searched = $entityManager->getRepository(Advert::class)->findBy(["address" => $addressIds]);

return $this->render("Search/searched.html.twig", ["searched" => $searched]);
}


return $this->render("Search/index.html.twig", ["form" => $form->createView()]);
}

Call to a member function map() on array

No i? Masz dokładny komunikat błędu. $address jest tablicą obiektów a nie obiektem typu Iterator.

Tak powinno wyglądać

[PHP] pobierz, plaintext 
$addressIds = array_map(function($single_address) {
    return $single_address->id;
}, $address);
[PHP] pobierz, plaintext