ORM won't kill any kittens

Last Friday of August I presented a session on the monthly meetup of my local user group Mexicali Open Source

The session name was "ORM won't kill any kittens". It was a few recomentations about how to use an ORM Doctrine in this particular case, since the demo was coded on Symfony2.

This was the topic list:

The Symfony WebProfiler
Write the DQL statements
Lazy Associations
Avoid Object Hydration and use Array Hydration instead
Avoid fetching all of the properties
Avoid loading entities use a reference instead
Batch Processing
Write DQL for Updates
The N+1 Problem

The final point "The N+1 Problem" was the main reason for this session, since a few sessions back another member of the comunity was presenting a demo about a web application and looking at the quantity of executed queries, I mentioned this seems to be the N+1 Problem but what is this.

This issue appear when you need to load the children of an entity-relation. The problem is one query is executed for the parent record and one query of each child record.

As you may expect instead of one single query a lot of queries are executed and that is something you should try to avoid.

Along with the presentation I did a demo of how to solve the "N+1 Problem" writing DQL statements and adding some code on the Controller.

This is the number of the pre-loaded records and the relation between the entities on the demo:

250 Articles
10 Categories (Many to One)
100 Tags (Many to Many)

Without writing any DQL/Query and using default methods provided by Doctrine to load the list of Article entities from the DB.


$articles = $entityManager->getRepository('ORMDemoBundle:Article')->findAll();

Twig template


{% for article in articles %}

  {{ article.title }}

  {{ article.category.name }}

  {% for tag in article.tags %}
    {{ tag.name }} {%- if loop.last == false -%}, {%- endif -%}
  {% endfor %}

{% endfor %}

Default behaviour: 511 DB Queries execution time 165.19 ms, 261 removing fetch="EXTRA_LAZY" and setting "EAGER" on Article association mapping Tag.

I will show the steps I used to approach this problem and reduce the number of executed queries to only 2.

Categories (Many to One)

The first improvement is create a DQL/Query and use LEFT JOIN on the relation between Article and Category.


public function findByArticleCategory(){

  $queryBuilder = $this->getQueryBuilder();
  $query = $queryBuilder->select('a.id, a.title, a.body, c.name as category_name')
                        ->from('ORMDemoBundle:Article','a')
                        ->leftJoin('a.category', 'c')
                        ->getQuery();

  return $query->getArrayResult();
}

Runnable SQL generated from Doctrine.


SELECT a0_.id AS id0, a0_.title AS title1, 
       a0_.body AS body2, c1_.name AS name3 
FROM article a0_ LEFT JOIN category c1_ ON a0_.category_id = c1_.id;

As simple as this looks it remove the need to execute 10 DB queries when executing the SQL statement.


//Obtain the list of Articles 
$articles = $entityManager->getRepository('ORMDemoBundle:Article')
                          ->findByArticleCategory();

Tags (Many to Many)

As you can imagine the following improvement is also related to creating a DQQuery, but first extract all of the Article's ids obtained on the previous query result, the array_column function was used for that purpose.


// Extract the values from the id column on the articles array
$articleKeys = array_column($articles, 'id');

This array of Article ids it's be pass as an argument to the next DQL/Query in this case using the relation between Tags & Articles to implement a LEFT JOIN on Articles and then filter the id column with the passed id values.


public function findTagsByArticle($articleKeys){

  $queryBuilder = $this->getQueryBuilder();
  $query = $queryBuilder->select('article.id as article_id,
                                  tag.id as tag_id, tag.name as tag_name')
                        ->from('ORMDemoBundle:Tag','tag')
                        ->leftJoin('tag.articles', 'article', 'ON')
                        ->where('article.id IN (:articleKeys)')
                        ->setParameter('articleIds', $articleKeys)
                        ->getQuery();

  return $query->getArrayResult();
}

Runnable SQL generated from Doctrine


SELECT a0_.id AS id0, t1_.id AS id1, t1_.name AS name2 
FROM tag t1_ 
LEFT JOIN articles_tags a2_ ON t1_.id = a2_.tag_id 
LEFT JOIN article a0_ ON a0_.id = a2_.article_id 
WHERE a0_.id IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 
16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 
33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 
50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 
67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 
84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 
101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 
115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 
129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 
143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 
157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 
171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 
185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 
199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 
213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 
227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 
241, 242, 243, 244, 245, 246, 247, 248, 249, 250);

Executing that will return an array of the matching relation between Tags & Articles.


$articles_tags = $entityManager->getRepository('ORMDemoBundle:Article')
                               ->findTagsByArticle($articleKeys);

Next use foreach to iterate the result and create a new array and concatenate all of the matching Tags by Article id.


$tags = [];
foreach ($articles_tags as $article_tag){
  $article_id = $article_tag['article_id'];
  if (array_key_exists($article_id, $tags)) {
    $tags[$article_id] = $tags[$article_id] . ', ' . $article_tag['tag_name'];
  }
  else {
    $tags[$article_id] = $article_tag['tag_name'];
  }
}

And finally the two arrays $articles & $tags are passed from the controller to the Twig file and use the for loop to display the results


{% for article in articles %}
  {{ article.title }}

  {{ article.category_name }}

  {{ tags[article.id] }}
{% endfor %}

Optimized behaviour: 2 DB Queries execution time 6.23 ms.

You can see the slides I used on the presentation here:
http://jmolivas.com/slides/mxlos/orm-wont-kill-any-kittens/

You can also take a look at the code used for the demo on this github repository:
https://github.com/jmolivas/orm.dev