N 1 query problems can be solved through Laravel's preload function. When an associated query is initiated separately for each record after obtaining the main table data, the number of database queries will surge, for example, 20 article data are retrieved but 21 queries are performed. Laravel uses the with() method to load the association model at one time, such as Post::with('user')->get() to control the query within two times. Post::with('user.role')->get() is available when handling multi-layer nested relationships. Restrictions can be passed in closures when loading fields or adding conditions, but foreign keys must be retained. Easily ignored points include Blade template access to unpreloaded properties, dynamic accessor call relationships, and using whereHas without cooperating with.
To solve the N 1 query problem, Laravel's preloading function is actually very critical.
Have you ever encountered this situation: Everything looks like it is normal when the page is loading, but the number of queries in the database is growing explosively? For example, if you fetch 20 article data, you found that 21 SQLs were executed in the background - this is a typical N 1 query problem . Don't worry, Laravel provides a simple and practical solution: Eager Loading .
What is an N 1 query problem?
Simply put, after you obtain the main table data, you initiate an association query for each record separately.
For example:
You have a Post model and a User model, and each post belongs to a user. When you write code like this:
$posts = Post::all();
foreach ($posts as $post) {
echo $post->user->name;
} Although only Post::all() is called, a new query is triggered every time you loop through $post->user . If there are 100 articles, this becomes 101 queries - that is, N 1.
How to solve this problem with Eager Loading in Laravel?
The answer is simple: use the with() method to load the association model at one time.
Or the above example, modified to:
$posts = Post::with('user')->get();
In this way, Laravel will first find out all articles, and then pull back all relevant user data through an additional query. No matter how many articles there are, there will be only two inquiries at most.
Note: Make sure that your relationship method is named correctly, such as defining
public function user()in thePostmodel.
How to deal with more complex scenarios?
Sometimes you will nest relationships at multiple levels, or you will need to filter associated data. At this time, you can use more advanced syntax:
Loading multi-layer nested relationships
For example, you want to load the article, author and author's role information at the same time:
Post::with('user.role')->get();
As long as your model relationship is set correctly, this writing method can help you solve the three-level nesting directly.
Load only some fields or add conditions
You can pass in closures to limit loaded data:
Post::with(['user' => function($query) {
$query->select('id', 'name');
}])->get();However, note that if you only select some fields, you must keep the foreign key (such as
user_id), otherwise the association cannot be matched correctly.
What are the places that are easy to ignore and cause the N 1 problem to recur?
-
Accessing unpreloaded relationship properties in Blade templates
Even if the controller uses
with(), a new query will still be triggered if you access a relationship that is not loaded in Blade. -
Relationships are called in dynamic accessor
If you write something like
getFullNameAttribute()in the model and use$this->relationin it, you may also quietly introduce additional queries. -
Use
whereHasbut notwithwhereHasis used to filter data and will not automatically load the corresponding relationship for you. Remember to addwith().
Basically that's it. The N 1 problem may seem small, but it can easily drag down performance; fortunately, Laravel's preloading mechanism is powerful enough and not complicated to use. The key is to develop the habit of confirming whether it has been preloaded before accessing the relationship, and avoid accidentally triggering multiple queries.
The above is the detailed content of The N 1 problem and Laravel Eager Loading.. For more information, please follow other related articles on the PHP Chinese website!
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