Querying Relationships in Laravel with whereRelation

Laravel's query builder provides the whereRelation method, which is very useful for querying relationships.

Here's the syntax for using the whereRelation method:

Model::whereRelation('relation', 'column', 'operator', 'value')
// or, using the default (=) operator
Model::whereRelation('relation', 'column', 'value')

Suppose there is an Order model that has a hasMany relationship with the OrderItem model:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Order extends Model
{

    public function items() {
        return $this->hasMany(OrderItem::class);
    }   

}

Now, let's say we want to get all orders that have items with a quantity greater than 100. This can be done using the whereRelation method. For example:

$ordersWithMinimumItemQty = Order::whereRelation('items', 'quantity', '>', 100)
    ->get();

Behind the scenes, the whereRelation method actually calls the whereHas method. For example:

$ordersWithMinimumItemQty = Order::whereRelation('items', 'quantity', '>', 100)
    ->get();
// Equivalent to using whereHas:
$ordersWithMinimumItemQty = Order::whereHas('items', function ($query) {
        $query->where('quantity', '>', 100);
    })->get();

As you can see, whereRelation simplifies the use of the whereHas method.