一对多关系船上的学说表现

Question

I am wondering whether OneToMany Realationship will hamper on application's performance. Say, city and user entity are in a OneToMany relationship, that means, a city can contain a lots of users. Now after setting the relationship in entity classes, whenever I retrieve a city, I can get its users via:

$users = $city->getUsers();

Now, I am wondering about the internal architechture. Whenever I retrieve city, will it query for all users also? If so, lets say a city can have 10000 users. Now won't it be a performance issue that I am retrieving a city only, but its getting along with all 10000 users as well? Or, it uses some other machanism and I am totally ok with it to implement?

Wish to get an explanation from you experts and suggestions about best practices regarding this situations as well. Thanks in advance.

Answer 1

Well, I am not an expert, but I can share you some good practices that you could consider at application development:

fetch="EXTRA_LAZY"

By default, doctrine 2.0 will load the entire collection and store it memory. In a scenario like yours, the users collection could be a performance problems by the table dimesion. So, why not mark the relationship as EXTRA_LAZY? An entity with this fetch mode only will be load when is accessed without triggering a full load of the collection.

/**
 * @ManyToMany(targetEntity="User", mappedBy="cities", fetch="EXTRA_LAZY")
 */

Allowing this fetch mode, you are able to make use of function like slide() and count() robustly. For example:

$users = $em->getRepository('models\User')->findAll();
echo  $users->count();

The code triggers a sql statement like select count(*) from users.. instead looping through the $user collection.

Hydrating objects

Not always is necessary to load a collection of entities. If you are creating a blog system, we need to provide only a list of post titles. This infrasrtucture can be inproved by Hydrating objects for read-only purposes.

Useful links

Some links that routes you to official doctrine docs guide.

• Improving performance
• Doctrine 2, best practices