Apply eloquent eager limit query using SQL ROW_NUMBER in Laravel

As per @Igor's response, I made it more handy by placing it in traits & local scopes.

App\Traits\WithEagerLimit.php

<?php

namespace App\Traits;

use Illuminate\Database\Eloquent\Builder;

trait WithEagerLimit
{
    public function scopeWithEagerLimit(Builder $builder, string $relation, callable $callback)
    {
        return $builder->with([ $relation => function($query) use ($builder, $callback) {

            $limit = $builder->getQuery()->limit ?: 1;

            // Illuminate\Database\Eloquent\Relations\Relation
            $query = call_user_func_array($callback, [$query]);

            // Illuminate\Database\Query\Builder
            $dbQB = $query->getQuery()->getQuery();

            if(!$dbQB->limit) {
                return $query;
            }

            $dbQB->limit *=  $limit;

            return $query->when(is_null($dbQB->columns), fn($q) => $q->select('*'))
                ->selectRaw("row_number() over (partition by ".$query->getExistenceCompareKey().") as laravel_row")
                ->orderBy('laravel_row');
        }]);
    }
}

Use the WithEagerLimit trait on the corresponding model.

For example:

class User extends Model {

    use \App\Traits\WithEagerLimit;
    
    //...

    public function posts()
    {
        return $this->hasMany(Post::class);
    }
}

Now, you can chain withEagerLimit() to any of your queries by doing the following:

Simple eager loading functionality

User::withEagerLimit('posts', fn($query) => $query)->get();

// SQL
select * from `users`

select * from `posts` where `posts`.`user_id` in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

Conditions on eager loading functionality

User::query()
    ->withEagerLimit('posts', fn($query) => $query->select(['id', 'user_id'])->where('id', '<=', 5))
->get();

// SQL
select * from `users`

select `id`, `user_id` from `posts` where `posts`.`user_id` in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) and `id` <= 5

Complex SQL RAW query on eager loading functionality

User::query()
    ->select('id')
    ->withEagerLimit('posts',
        fn($query) => $query->select(['id', 'user_id'])
            ->whereRaw('MOD(id, 2) = 0')
            ->limit(4)
    )
    ->whereRaw('MOD(id, 2) = 1')
    ->skip(3)
    ->take(2)
    ->latest('id')
->get();

// SQL
select `id` from `users` where MOD(id, 2) = 1 order by `id` desc limit 2 offset 3

select `id`, `user_id`, row_number() over (partition by posts.user_id) as laravel_row from `posts` where `posts`.`user_id` in (1, 3) and MOD(id, 2) = 0 order by `laravel_row` asc limit 8

Pagination on eager loading functionality

User::query()
    ->withEagerLimit('posts', fn($query) => $query->limit(4))
->paginate(5);

// SQL
select count(*) as aggregate from `users`

select * from `users` limit 5 offset 0

select *, row_number() over (partition by posts.user_id) as laravel_row from `posts` where `posts`.`user_id` in (1, 2, 3, 4, 5) order by `laravel_row` asc limit 20

For those eagerly waiting for an answer

You should call ->limit($value) or an equivalent method (i.e. take(), skip(), paginate(), or similar procedure) in any queries chained with ->withEagerLimit($relationName, $callback) in the main-query and also in sub-query.

User::query()
    ->select('id')
    ->withEagerLimit('posts', fn($query) => $query->select(['id', 'user_id'])->limit(4))
    ->limit(2)
->get();

You get the benefits of using this trait only when the query meets enough results on both the tables i.e. users & posts. Otherwise, you will get extra results out of the box because here we have done UsersLimit x PostsLimit, so keep the query accordingly.