Default API Filters Failing when referencing nested properties of related entities - symfony

Question

I am configuring a series of ApiFilter classes from the Api-platform stack in a Symfony 4 application. I am finding that any filters using a nested property from an entity relationship takes no effect. Please see my example code below, would really appreciate any assistance with why it may not be working.
The entity I am trying to filter on is:
<?php
namespace App\Entity;
use ApiPlatform\Core\Annotation\ApiResource;
use Doctrine\ORM\Mapping as ORM;
use ApiPlatform\Core\Annotation\ApiProperty;
/**
* PostAuthor
* #ApiResource()
* #ORM\Table(name="POST_AUTHOR", indexes={#ORM\Index(name="IDX_CF3397D94A7000A0", columns={"IDENTITY_ID"})})
* #ORM\Entity(repositoryClass="App\Repository\PostAuthorRepository")
*/
class PostAuthor
{
/**
* #var string
*
* #ORM\Column(name="USER_ID", type="string", length=255, nullable=false)
* #ORM\Id
*/
private $userId;
/**
* #var string
*
* #ORM\Column(name="USERNAME", type="string", length=255, nullable=false)
*/
private $username;
And the collection I am filtering is:
<?php
namespace App\Entity;
use Doctrine\Common\Collections\ArrayCollection;
use ApiPlatform\Core\Annotation\ApiResource;
use ApiPlatform\Core\Annotation\ApiFilter;
use Symfony\Component\Serializer\Annotation\Groups;
use ApiPlatform\Core\Bridge\Doctrine\Orm\Filter\SearchFilter;
use ApiPlatform\Core\Bridge\Doctrine\Orm\Filter\NumericFilter;
use Doctrine\ORM\Mapping as ORM;
/**
* PostHeader
* #ORM\Table(name="POST_HEADER", indexes={#ORM\Index(name="IDX_1CEEE7D0B65E5FF8", columns={"EMOTION_REF_ID"}), #ORM\Index(name="IDX_1CEEE7D08AFAAB14", columns={"AUTHOR_ID"})})
* #ORM\Entity(repositoryClass="App\Repository\PostHeaderRepository")
* #ApiResource(
* normalizationContext={"groups"={"postheader:read"}},
* denormalizationContext={"groups"={"postheader:write"}},
* )
* #ApiFilter(SearchFilter::class, properties={"author.userId": "partial", "author.username": "partial"})
*/
class PostHeader
{
/**
* #var PostAuthor
*
* #ORM\ManyToOne(targetEntity="PostAuthor")
* #ORM\JoinColumns({#ORM\JoinColumn(name="AUTHOR_ID", referencedColumnName="USER_ID")})
* #Groups({"postheader:read"})
*/
private $author;

Answer 1

I just test on a new project with the same classes than you and it works. I can filter by author.userId.
Please give us your testing request. And the error message ;).