Lazy loading and N+1 prevention in Laravel - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output count of queries executed?

Given the following Laravel Eloquent code, how many database queries will be executed?

use App\Models\Post;

$posts = Post::all();
foreach ($posts as $post) {
    echo $post->comments->count();
}

Laravel

use App\Models\Post;

$posts = Post::all();
foreach ($posts as $post) {
    echo $post->comments->count();
}

A N+1 queries, where N is the number of posts

B 1 query

C N queries, where N is the number of posts

D 2 queries

💡 Hint

Think about how Laravel loads related models when accessed without eager loading.

The code first fetches all posts in one query. Then, for each post, accessing comments triggers a separate query. This results in 1 query for posts plus N queries for comments, totaling N+1 queries.

📝 Syntax

intermediate

Which code snippet correctly prevents N+1 problem using eager loading?

Select the Laravel Eloquent code that correctly uses eager loading to prevent the N+1 query problem when retrieving posts with their comments.

A $posts = Post::with('comments')->get();

B $posts = Post::all()->load('comments');

C $posts = Post::get()->with('comments');

D $posts = Post::withComments()->get();

💡 Hint

Eager loading is done before retrieving the data, not after.

Option A uses with('comments') before get(), which eager loads comments in one query. Option A calls load() on a collection, which works but is less efficient than eager loading before retrieval. Option A is invalid syntax because with() cannot be called after get(). Option A uses a non-existent method.

🔧 Debug

advanced

Why does this eager loading code still cause N+1 queries?

Consider this Laravel code:

$posts = Post::with('comments')->get();
foreach ($posts as $post) {
    echo $post->comments()->where('approved', true)->count();
}

Why does this still cause N+1 queries?

Laravel

$posts = Post::with('comments')->get();
foreach ($posts as $post) {
    echo $post->comments()->where('approved', true)->count();
}

A Because with('comments') does not eager load comments

B Because comments() is called as a query each time, ignoring eager loaded data

C Because count() cannot be used on eager loaded relationships

D Because the where clause is invalid syntax

💡 Hint

Check how the comments relationship is accessed inside the loop.

Using comments() calls the relationship method returning a query builder, so each count() triggers a new query. The eager loaded comments are accessed via the property comments without parentheses.

🧠 Conceptual

advanced

How to eager load nested relationships to prevent N+1?

You want to load posts with their comments and each comment's author to avoid N+1 queries. Which eager loading syntax is correct?

A $posts = Post::with('comments')->with('author')->get();

B $posts = Post::with(['comments', 'author'])->get();

C $posts = Post::with('comments.author')->get();

D $posts = Post::with('comments')->load('author');

💡 Hint

Nested relationships are specified with dot notation inside with().

Option C correctly eager loads comments and their author in one query. Option C tries to load author directly on posts, which is incorrect. Option C loads author on posts separately, not nested. Option C uses load() after get(), which is less efficient.

❓ state_output

expert

What is the output count of queries executed with conditional eager loading?

Given this Laravel code:

$posts = Post::with(['comments' => function ($query) {
    $query->where('approved', true);
}])->get();

foreach ($posts as $post) {
    echo $post->comments->count();
}

How many queries are executed?

Laravel

$posts = Post::with(['comments' => function ($query) {
    $query->where('approved', true);
}])->get();

foreach ($posts as $post) {
    echo $post->comments->count();
}

A 1 query

B N+1 queries, where N is the number of posts

C 3 queries

D 2 queries

💡 Hint

Constrained eager loading performs 1 query for posts and 1 query for filtered comments.

The with() method with a closure applies a condition to eager load only approved comments. This results in 1 query for posts + 1 query for approved comments, totaling 2 queries. Accessing $post->comments uses the already loaded data, so only 2 queries total.