How to Select Specific Columns from Related Tables Using Eloquent's `with()` Function?

Retrieving Specific Columns from Related Tables with Eloquent's "With()" Function

When performing joins using Eloquent's "with()" function, it is possible to retrieve specific columns from the related table(s). This is useful in scenarios where only a subset of columns is required from the joined table.

Problem:

Consider a scenario with two tables, User and Post, where one User can have multiple posts and each post belongs to a single user.

Initial Approach:

Using the default "with('user')" syntax, the following queries are executed:

select * from `posts`
select * from `users` where `users`.`id` in (<1>, <2>)

However, if only specific columns are required from the "users" table, such as "id" and "username", the above approach will not suffice.

Solution I: Using an Array as the Second Argument in "with()":

Passing an array as the second argument in "with()" allows you to specify which columns to retrieve from the related table. However, this approach only works for specifying columns from the first table.

Solution II: Using a Closure as the Second Argument in "with()":

To retrieve specific columns from the second table, use a closure function as the second argument in the "with()" array. This technique enables you to implement custom logic for selecting columns.

Post::query()
    ->with(['user' => function ($query) {
        $query->select('id', 'username');
    }])
    ->get()

With this approach, the following queries are executed:

select * from `posts`
select id, username from `users` where `users`.`id` in (<1>, <2>)

Note: The primary key of the related table ("id" in this example) must be the first parameter in $query->select() to retrieve the desired results.

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