How does Laravel handle foreign keys in nested eager loading

Eager Loading Nested Relationships

To eager load nested relationships in Laravel, you can use the "dot" notation to specify the nested relationships you want to load. For example, to eager load a book's author and the author's contacts in one Eloquent statement:

php
$books = App\Book::with('author.contacts')->get();

Eager Loading Specific Columns

When eager loading relationships, you can specify which columns you want to retrieve for each relationship. This is important when dealing with nested relationships, as you need to include the foreign key columns:

php
$books = App\Book::with('author:id,name', 'author.contacts:id,phone')->get();

In this example, we're only retrieving the `id` and `name` columns from the `authors` table, and the `id` and `phone` columns from the `contacts` table.

Importance of Foreign Key Columns

The key thing to note is that when eager loading nested relationships, you must include the foreign key columns. Otherwise, the nested models may not load at all.

For example, if you have a `Book` model that belongs to an `Author`, and the `Author` model has a `Contact` model, you would need to include the `author_id` column when eager loading the `contacts` relationship:

php
$books = App\Book::with('author.contacts:id,phone,author_id')->get();

If you don't include the `author_id` column, the `contacts` relationship may not load properly.

Constraining Eager Loads

You can also add constraints to the eager loading queries, just like you can with regular Eloquent queries:

php
$books = App\Book::with(['author' => function ($query) {
    $query->where('name', 'like', '%John%');
}, 'author.contacts' => function ($query) {
    $query->where('phone', 'like', '%555%');
}])->get();

This will only eager load authors whose name contains "John", and contacts whose phone number contains "555".

In summary, when eager loading nested relationships in Laravel, it's crucial to include the necessary foreign key columns to ensure the nested models are properly loaded. Failing to do so can result in incomplete or missing data.