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mirror of synced 2024-12-14 23:26:04 +03:00

Updated documentation on chosing inverse-owning side for many-to-many associations and came up with a significantly enhanced example for the working with associations chapter

This commit is contained in:
Benjamin Eberlei 2010-07-23 23:21:17 +02:00
parent 298773b2f8
commit 62c4f3e6fb
2 changed files with 269 additions and 52 deletions

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@ -573,6 +573,46 @@ Here is a similar many-to-many relationship as above except this one is bidirect
The MySQL schema is exactly the same as for the Many-To-Many uni-directional case above.
+++ Picking the Owning and Inverse Side
For Many-To-Many associations you can chose which entity is the owning and which the inverse side. There is
a very simple semantic rule to decide which side is more suitable to be the owning side from a developers perspective.
You only have to ask yourself, which entity is responsible for the connection management and pick that as the owning side.
Take an example of two entities `Article` and `Tag`. Whenever you want to connect an Article to a Tag and vice-versa, it is mostly
the Article that is responsible for this relation. Whenever you add a new article, you want to connect it with existing or new tags.
Your create Article form will probably support this notion and allow to specify the tags directly. This is why you should
pick the Article as owning side, as it makes the code more understandable:
[php]
class Article
{
private $tags;
public function addTag(Tag $tag)
{
$tag->addArticle($this); // synchronously updating inverse side
$this->tags[] = $tag;
}
}
class Tag
{
private $articles;
public function addArticle(Article $article)
{
$this->articles[] = $article;
}
}
This allows to group the tag adding on the `Article` side of the association:
[php]
$article = new Article();
$article->addTag($tagA);
$article->addTag($tagB);
++ Many-To-Many, Self-referencing
You can even have a self-referencing many-to-many association. A common scenario is where a `User` has friends and the target entity of that relationship is a `User` so it is self referencing. In this example it is bidirectional so `User` has a field named `$friendsWithMe` and `$myFriends`.

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@ -180,29 +180,168 @@ and you want to modify and persist such an entity.
Associations between entities are represented just like in regular object-oriented PHP, with references to other objects or collections of objects. When it comes to persistence, it is important to understand three main things:
* The concept of owning and inverse sides in bidirectional associations as described [here](http://www.doctrine-project.org/documentation/manual/2_0/en/association-mapping#owning-side-and-inverse-side).
* A collection of entities always only represents the association to the containing entities. If an entity is removed from a collection, the association is removed, not the entity itself.
* Collection-valued persistent fields and properties must be defined in terms of the Doctrine\Common\Collections\Collection interface. [See here](http://www.doctrine-project.org/documentation/manual/2_0/en/architecture#entities:persistent-fields) for more details.
* If an entity is removed from a collection, the association is removed, not the entity itself. A collection of entities always only represents the association to the containing entities, not the entity itself.
* Collection-valued persistent fields have to be instances of the `Doctrine\Common\Collections\Collection` interface. [See here](http://www.doctrine-project.org/documentation/manual/2_0/en/architecture#entities:persistent-fields) for more details.
+++ Example Associations
We will use a simple comment system with Users and Comments as entity to show examples of association management:
[php]
/** @Entity */
class User
{
/** @Id @GeneratedValue @Column(type="string") */
private $id;
/**
* Bidirectional - Many users have Many favorite comments (OWNING SIDE)
*
* @ManyToMany(targetEntity="Comment", inversedBy="userFavorites")
* @JoinTable(name="user_favorite_comments",
* joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
* inverseJoinColumns={@JoinColumn(name="favorite_comment_id", referencedColumnName="id")}
* )
*/
private $favorites;
/**
* Unidirectional - Many users have marked many comments as read
*
* @ManyToMany(targetEntity="Comment")
* @JoinTable(name="user_read_comments",
* joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
* inverseJoinColumns={@JoinColumn(name="comment_id", referencedColumnName="id")}
* )
*/
private $commentsRead;
/**
* Bidirectional - One-To-Many (INVERSE SIDE)
*
* @OneToMany(targetEntity="Comment", mappedBy="author")
*/
private $commentsAuthored;
/**
* Unidirectional - Many-To-One
*
* @ManyToOne(targetEntity="Comment")
*/
private $firstComment;
}
/** @Entity */
class Comment
{
/** @Id @GeneratedValue @Column(type="string") */
private $id;
/**
* Bidirectional - Many comments are favorited by many users (INVERSE SIDE)
*
* @ManyToMany(targetEntity="User", mappedBy="favorites")
*/
private $userFavorites;
/**
* Bidirectional - Many Comments are authored by one user (OWNING SIDE)
*
* @ManyToOne(targetEntity="User", inversedBy="authoredComments")
*/
private $author;
}
This two entities generate the following MySQL Schema (Foreign Key definitions omitted):
[sql]
CREATE TABLE User (
id VARCHAR(255) NOT NULL,
firstComment_id VARCHAR(255) DEFAULT NULL,
PRIMARY KEY(id)
) ENGINE = InnoDB;
CREATE TABLE Comment (
id VARCHAR(255) NOT NULL,
author_id VARCHAR(255) DEFAULT NULL,
PRIMARY KEY(id)
) ENGINE = InnoDB;
CREATE TABLE user_favorite_comments (
user_id VARCHAR(255) NOT NULL,
favorite_comment_id VARCHAR(255) NOT NULL,
PRIMARY KEY(user_id, favorite_comment_id)
) ENGINE = InnoDB;
CREATE TABLE user_read_comments (
user_id VARCHAR(255) NOT NULL,
comment_id VARCHAR(255) NOT NULL,
PRIMARY KEY(user_id, comment_id)
) ENGINE = InnoDB;
++ Establishing Associations
Establishing an association between two entities is straight-forward. Here are some examples:
Establishing an association between two entities is straight-forward. Here are some examples for the unidirectional
relations of the `User`:
[php]
// Article <- one-to-many -> Comment
$article->getComments()->add($comment);
$comment->setArticle($article);
class User
{
// ...
public function getReadComments() {
return $this->commentsRead;
}
// User <- many-to-many -> Groups
$user->getGroups()->add($group);
$group->getUsers()->add($user);
public function setFirstComment(Comment $c) {
$this->firstComment = $c;
}
}
// User <- one-to-one -> Address
$user->setAddress($address);
$address->setUser($user);
// unidirectional many to many
$user->getReadComments()->add($comment);
// unidirectional many to one
$user->setFirstComment($myFirstComment);
In the case of bi-directional associations you have to update the fields on both sides:
[php]
class User
{
// ..
public function getAuthoredComments() {
return $this->commentsAuthored;
}
public function getFavoriteComments() {
return $this->favorites;
}
}
class Comment
{
// ...
public function getUserFavorites() {
return $this->userFavorites;
}
public function setAuthor(User $author = null) {
$this->author = $author;
}
}
// Many-to-Many
$user->getFavorites()->add($favoriteComment);
$favoriteComment->getUserFavorites()->add($user);
// Many-To-One / One-To-Many Bidirectional
$user->getAuthoredComments()->add($newComment);
$newComment->setAuthor($user);
Notice how always both sides of the bidirectional association are updated. Unidirectional associations are consequently simpler to handle.
Notice how always both sides of the bidirectional association are updated. The previous unidirectional associations were simpler to handle.
++ Removing Associations
@ -211,30 +350,31 @@ to do so, by key and by element. Here are some examples:
[php]
// Remove by Elements
// Article <- one-to-many -> Comment
$article->getComments()->removeElement($comment);
$comment->setArticle(null);
$user->getComments()->removeElement($comment);
$comment->setAuthor(null);
// User <- many-to-many -> Group
$user->getGroups()->removeElement($group);
$group->getUsers()->removeElement($user);
$user->getFavorites()->removeElement($comment);
$comment->getUserFavorites()->removeElement($user);
// Remove by key
$article->getComments()->remove($ithComment);
$comment->setArticle(null);
// Remove by Key
$user->getComments()->removeElement($ithComment);
$comment->setAuthor(null);
// User <- one-to-one -> Address
$user->setAddress(null);
$address->setUser(null);
Notice how both sides of the bidirectional association are always updated. Unidirectional associations are consequently simpler to handle. Also note that if you type-hint your methods, i.e. `setAddress(Address $address)`, then PHP does not allow null values and setAddress(null) will fail for removing the association. If you insist on type-hinting a typical way to deal with this is to provide a special method, like `removeAddress()`. This can also provide better encapsulation as it hides the internal meaning of not having an address.
Notice how both sides of the bidirectional association are always updated. Unidirectional associations are consequently
simpler to handle. Also note that if you type-hint your methods, i.e. `setAddress(Address $address)`, then PHP does only
allows null values if `null` is set as default value. Otherwise setAddress(null) will fail for removing the association.
If you insist on type-hinting a typical way to deal with this is to provide a special method, like `removeAddress()`.
This can also provide better encapsulation as it hides the internal meaning of not having an address.
When working with collections, keep in mind that a Collection is essentially an ordered map (just like a PHP array).
That is why the `remove` operation accepts an index/key. `removeElement` is a separate method
that has O(n) complexity, where n is the size of the map.
that has O(n) complexity using `array_search`, where n is the size of the map.
Since Doctrine always only looks at the owning side of a bidirectional association, it is essentially not necessary that an inverse collection of a bidirectional one-to-many or many-to-many association is updated. This knowledge can often be used to improve performance by avoiding the loading of the inverse collection.
> **NOTE**
>
> Since Doctrine always only looks at the owning side of a bidirectional association for updates, it is not necessary
> for write operations that an inverse collection of a bidirectional one-to-many or many-to-many association is updated.
> This knowledge can often be used to improve performance by avoiding the loading of the inverse collection.
> **NOTE*
>
@ -250,38 +390,75 @@ Since Doctrine always only looks at the owning side of a bidirectional associati
It is generally a good idea to encapsulate proper association management inside the entity classes. This makes it easier to use the class correctly and can encapsulate details about how the association is maintained.
The following code shows a simple, idiomatic example for a bidirectional one-to-many association between an Article and its Comments.
The following code shows updates to the previous User and Comment example that encapsulate much of
the association management code:
[php]
// Mappings not shown.
class Article {
// The comments of the article.
private $comments;
// ... constructor omitted ...
class User
{
//...
public function markCommentRead(Comment $comment) {
// Collections implement ArrayAccess
$this->commentsRead[] = $comment;
}
public function addComment(Comment $comment) {
$this->comments->add($comment);
$comment->setArticle($this);
if (count($this->commentsAuthored) == 0) {
$this->setFirstComment($comment);
}
public function getComments() {
return $this->comments;
$this->comments[] = $comment;
$comment->setAuthor($this);
}
private function setFirstComment(Comment $c) {
$this->firstComment = $c;
}
class Comment {
// The article the comment refers to.
private $article;
// ... constructor omitted ...
public function setArticle($article) {
$this->article = $article;
public function addFavorite(Comment $comment) {
$this->favorites->add($comment);
$comment->addUserFavorite($this);
}
public function getArticle() {
return $this->article;
public function removeFavorite(Comment $comment) {
$this->favorites->removeElement($comment);
$comment->removeUserFavorite($this);
}
}
With the above implementation, it is always ensured that at least the owning side from Doctrine's point of view (Comment) is properly updated. You will notice that `setArticle` does not call `addComment`, thus the bidirectional association is strictly-speaking still incomplete, if a user of the class only invokes `setArticle`. If you naively call `addComment` in `setArticle`, however, you end up with an infinite loop, so more work is needed. As you can see, proper bidirectional association management in plain OOP is a non-trivial task and encapsulating all the details inside the classes can be challenging.
class Comment
{
// ..
There is no single, best way for association management. It greatly depends on the requirements of your concrete domain model as well as your preferences.
public function addUserFavorite(User $user) {
$this->userFavorites[] = $user;
}
public function removeUserFavorite(User $user) {
$this->userFavorites->removeElement($user);
}
}
You will notice that `addUserFavorite` and `removeUserFavorite` do not call `addFavorite` and `removeFavorite`,
thus the bidirectional association is strictly-speaking still incomplete. However if you would naively add the
`addFavorite` in `addUserFavorite`, you end up with an infinite loop, so more work is needed.
As you can see, proper bidirectional association management in plain OOP is a non-trivial task
and encapsulating all the details inside the classes can be challenging.
> **NOTE**
>
> If you want to make sure that your collections are perfectly encapsulated you should not return
> them from a `getCollectionName()` method directly, but call `$collection->toArray()`. This way a client programmer
> for the entity cannot circumvent the logic you implement on your entity for association management. Example:
>
> [php]
> class User {
> public function getReadComments() {
> return $this->commentsRead->toArray();
> }
> }
There is no single, best way for association management. It greatly depends on the requirements of your concrete
domain model as well as your preferences.
++ Transitive persistence