Doctrine2 (Doctrine 2.1) eager loading in Symfony2

问题:

Let's say I have two entities in my Symfony2 project : Category and Article (a category having many articles).

In my CategoryRepository, I have this method:

findAllDummy(){
  return $this->createQueryBuilder('c')
              ->leftJoin('c.Articles a')
              ->getQuery()->getResult();
}

If I remember well, in Symfony1.4 (and the corresponding version of Doctrine), the returned objects would have their 'articles' attribute filled by the corresponding Article objects. Now, in Symfony2, Proxy objects are returned.

So if I loop through a specific category's articles, As many queries as iterations will be executed.

foreach($category->getArticles() as $article){
  echo $article->getDoctrine()
               ->getRepository('')getTitle();
}

I understand this is Doctrine2.1's default lazy loading behavior.

Question 1: how is this a better solution? N queries instead of 1.

I tried to force eager loading by doing the following:

findAllDummy(){
  return $this->createQueryBuilder('c')
              ->leftJoin('c.articles a')
              ->getQuery()
              ->setFetchMode('Category', 'articles', 'EAGER')
              ->getResult();
}

But the result remains the same.

Question 2: how to force eager loading in Doctrine2?

回答1:

You're joining a table but you're not selecting anything from it. Add ->addSelect('a') to your query builder. Consider two following SQL queries to understand the difference:

SELECT a.id, a.title
FROM article a 
JOIN category c ON a.category_id = c.id 
WHERE a.id = 123;

SELECT a.id, a.title, c.id, c.name 
FROM article a 
JOIN category c ON a.category_id = c.id 
WHERE a.id = 123;

---

Eager/lazy joining has nothing to do with DQL queries. It defines what should be loaded when you use $articleRepository->find(123).

回答2:

In the part where you try to "force eager loading" the problem might be that you use the fetchMode method with the wrong variable type for the $fetchMode argument. You pass a string 'EAGER' but the method doesn't expect a string but an integer.

The method expects constants from the ClassMetadata class:

/**
 * Specifies that an association is to be fetched when it is first accessed.
 */
const FETCH_LAZY = 2;

/**
 * Specifies that an association is to be fetched when the owner of the
 * association is fetched.
 */
const FETCH_EAGER = 3;

In the Doctrine documentation chapter 14.7.6.6. Temporarily change fetch mode in DQL you can see an example on how to use this:

$query->setFetchMode("MyProject\User", "address", \Doctrine\ORM\Mapping\ClassMetadata::FETCH_EAGER);

So pass either a reference to the constant or an integer that corresponds to the mode you want to use.

回答3:

As it says in the doctrine docs, eager loading in this case won't make any difference because you have one-to-many relationship between Category and Article.

For one-to-many relations, changing the fetch mode to eager will cause to execute one query for every root entity loaded. This gives no improvement over the lazy fetch mode which will also initialize the associations on a one-by-one basis once they are accessed.

So contrary to what @Crozin has said, you can still do eager loading in DQL. If you have a one-to-one or many-to-one relationship, eager loading will solve the problem of making extra queries. However to solve your problem in this case you should use ->addSelect('a') as @Crozin mentioned.

回答4:

It is a better solution because joins are a much more expensive process than a simple query. While it may seem inefficient, it isn't much of a waste, and quickly becomes more efficient when you aren't loading every bit of every related object.