Doctrine Cascade Options for OneToMany - doctrine-orm

Question

I'm having a hard time making sense of the Doctrine manual's explanation of cascade operations and need someone to help me understand the options in terms of a simple ManyToOne relationship.
In my application, I have a table/entity named Article that has a foreign key field referencing the 'id' field in a table/entity named Topic.
When I create a new Article, I select the Topic from a dropdown menu. This inserts an integer into the 'topic_id' foreign key field in the Article table.
I have the $topic association set up in the Article entity like this:
/**
* #ManyToOne(targetEntity="Topic")
* #JoinColumn(name="topic_id", referencedColumnName="id", nullable=false)
*/
private $topic;
The Topic entity doesn't have any reciprocating annotation regarding the Article entity. Topics don't care what Articles reference them and nothing needs to happen to a Topic when an Article that references the Topic is deleted.
Because I'm not specifying the cascade operation in the Article entity, Doctrine throws an error when I try to create a new Article: "A new entity was found through a relationship that was not configured to cascade persist operations. Explicitly persist the new entity or configure cascading persist operations on the relationship."
So I know I need to choose a cascade operation to include in the Article entity, but how do I know which operation to choose in this situation?
From reading the Doctrine manual, "detach" sounds like the right option. But researching others' similar questions here and here makes me think I want to use "persist" instead.
Can anyone help me understand what "persist," "remove," "merge," and "detach" mean in terms of a simple ManyToOne relationship like the one I've described?

Answer 1

In the Doctrine2 documentation "9.6. Transitive persistence / Cascade Operations" there are few examples of how you should configure your entities so that when you persist $article, the $topic would be also persisted. In your case I'd suggest this annotation for Topic entity:
/**
* #OneToMany(targetEntity="Article", mappedBy="topic", cascade={"persist", "remove"})
*/
private $articles;
The drawback of this solution is that you have to include $articles collection to Topic entity, but you can leave it private without getter/setter.
And as #kurt-krueckeberg mentioned, you must pass the real Topic entity when creating new Article, i.e.:
$topic = $em->getRepository('Entity\Topic')->find($id);
$article = new Article($topic);
$em->persist($article);
$em->flush();
// perhaps, in this case you don't even need to configure cascade operations
Good luck!

Answer 2

If you have a #OneToMany unidirectional association, like that described in section 6.10 of the Doctrine Reference, then most likely you forgot to persist the Topic before calling flush. Don't set the topic_id primary key in Article. Instead set the Topic instance.
For example, given Article and Topic entities like these:
<?php
namespace Entities;
/**
#Entity
#Table(name="articles")
*/
class Article {
/**
* #Id
* #Column(type="integer", name="article_id")
* #GeneratedValue
*/
protected $id;
/**
* #Column(type="text")
*/
protected $text;
/**
* #ManyToOne(targetEntity="Topic", inversedBy="articles")
* #JoinColumn(name="topic_id", referencedColumnName="topic_id")
*/
protected $topic;
public function __construct($text=null)
{
if (!is_null($text)) {
$this->text = $text;
}
}
public function setArticle($text)
{
$this->text = $text;
}
public function setTopic(Topic $t)
{
$this->topic = $t;
}
}
<?php
namespace Entities;
/**
#Entity
#Table(name="topics")
*/
class Topic {
/**
* #Id
* #Column(type="integer", name="topic_id")
* #GeneratedValue
*/
protected $id;
public function __construct() {}
public function getId() {return $this->id;}
}
After you generate the schema:
# doctrine orm:schema-tool:create
your code to persist these entities would look like something this
//configuration omitted..
$em = \Doctrine\ORM\EntityManager::create($connectionOptions, $config);
$topic = new Entities\Topic();
$article1 = new Entities\Article("article 1");
$article2 = new Entities\Article("article 2");
$article1->setTopic($topic);
$article2->setTopic($topic);
$em->persist($article1);
$em->persist($article2);
$em->persist($topic);
try {
$em->flush();
} catch(Exception $e) {
$msg= $e->getMessage();
echo $msg . "<br />\n";
}
return;
I hope this helps.