# Belongs To Through Relations in Laravel 9

Take the following 3 models:

```xml
customer
    id - integer
    name - string
 
invoices
    id - integer
    status - string
    customer_id - integer
 
credit_notes
    id - integer
    status - string
    invoice_id - integer
```

Laravel gives us an easy way to get all [Credit Notes](https://en.wikipedia.org/wiki/Credit_note) that belong to a particular Customer through the invoice table. What you need to do, is to define a `hasManyThrough` relationship on the Customer model, as in:

```php
class Customer extends Model
{
    /**
     * Get all credit notes of the customer
     */
    public function creditNotes()
    {
        // we could call the below function with just first 2 arguments,
        // but I like being explicit when defining 'through' relations.    
        return $this->hasManyThrough(
            CreditNote::class,
            Invoice::class,
            'customer_id',
            'invoice_id',
            'id',
            'id'
        );
    }
}
```

What if we want to find out which Customer a particular Credit Note belongs to? We could call `$creditNote->invoice->customer`, but what if we don't want to load the Invoice model in the process? We can use the `hasOneThrough` relationship that can also act as a "belongs to through" relationship. The "trick" is to use custmomize arguments 3-6 of the relationship as below:

```php
class CreditNote extends Model
{
    /**
     * Get the customer for whom the credit note was issued.
     */
    public function customer()
    {
        return $this->hasOneThrough(
            Customer::class,
            Invoice::class,
            'id', // refers to id column on invoices table
            'id', // refers to id column on customers table
            'invoice_id', // refers to invoice_id column on credit_notes table
            'customer_id' // refers to customer_id column on invoices table
        );
    }
}
```

## Why do we need "Belongs To Through" relationships?

In my case, I use this "trick" a lot when I need to eager-load deep relationship, as in:

```php
App\CreditNote::all()->load('customer');
```

This saves us from unnecessarily loading Invoices associated with each Credit Note.

So far, I haven't run into any issues with defining "Belongs To Through" relationship this way, but since it's not something that you can find in the official Laravel Docs, I supposed there may be some "gotchas". If you have run into any of them, please let me know in the comments below.