ManyToMany relation is not persisted - php

Question

I have Post And PostTag entities. Only custom users are allowed create PostTags. While creating a post user may select post tags.
I have created many to many relationship between Post And PostTag entities using the make:entity command.
The problem is that after creating the post and attaching to it the selected tags the relation table is empty and nothing is returned by post.getPostTags() method.
PostController - create method:
...
$em = $this->getDoctrine()->getManager();
$post = $form->getData();
$post->setAuthor($this->getUser());
$post->setCreatedAt(new DateTime);
foreach ($form->get('post_tags')->getData() as $postTag) {
$post->addPostTag($postTag);
}
$em->persist($post);
$em->flush();
...
Post entity:
/**
* #ORM\ManyToMany(targetEntity="App\Entity\PostTag", mappedBy="post", cascade={"persist"})
*/
private $postTags;
public function __construct()
{
$this->postTags = new ArrayCollection();
}
/**
* #return Collection|PostTag[]
*/
public function getPostTags(): Collection
{
return $this->postTags;
}
public function addPostTag(PostTag $postTag): self
{
if (!$this->postTags->contains($postTag)) {
$this->postTags[] = $postTag;
$postTag->addPost($this);
}
return $this;
}
public function removePostTag(PostTag $postTag): self
{
if ($this->postTags->contains($postTag)) {
$this->postTags->removeElement($postTag);
$postTag->removePost($this);
}
return $this;
}
PostTag entity:
/**
* #ORM\ManyToMany(targetEntity="App\Entity\Post", inversedBy="postTags", cascade={"persist"})
*/
private $post;
public function __construct()
{
$this->post = new ArrayCollection();
}
/**
* #return Collection|Post[]
*/
public function getPost(): Collection
{
return $this->post;
}
public function addPost(Post $post): self
{
if (!$this->post->contains($post)) {
$this->post[] = $post;
}
return $this;
}
public function removePost(Post $post): self
{
if ($this->post->contains($post)) {
$this->post->removeElement($post);
}
return $this;
}

Answer 1

You have set the PostTag entity as the owning side (by virtue of setting inversedBy parameter on that entity), so you have to persist the PostTag entity, not the Post entity, to make things stick. You can make a simple modification to your Post entity to make this work:
public function addPostTag(PostTag $postTag): self
{
if (!$this->postTags->contains($postTag)) {
$this->postTags[] = $postTag;
$postTag->addPost($this);
// set the owning side of the relationship
$postTag->addPost($this);
}
return $this;
}