conditional relationship for "with()" - laravel

Question

I have a table called transactions and another table called cars with the below structure:
transactions
| id | date | amount | status | user_added |
| --- | ---- | ------ | ------ | ---------- |
cars
| id | plate | specs | buy_transaction | sell_transaction |
| --- | ----- | ----- | ---------------- | ----------------- |
A car has always a buy_transaction but not always a sell_transaction, situation is that I am trying to get all transactions (that might be car-related or not car-related) and include the CAR related to that transaction weather it is sold or bought, so I need to make the relationship conditional but i couldn't achieve that.
$journal = Transaction::with(
['user'=> function($query) {
$query->select('id', 'name');
},
'income',
'outcome',
'car'
])->where('date', '>=', $fromDate)->where('date', '<=', $toDate);
This is the modal class:
<?php
namespace App;
use Illuminate\Database\Eloquent\Model;
class Transaction extends Model
{
public function income()
{
//.....
}
public function outcome()
{
//.....
}
public function user()
{
return $this->belongsTo('App\User', 'user_added', 'id');
}
// *** problem starts here ***
public function car()
{
if (transaction status == 1) {
return $this->belongsTo('App\Car', 'id', 'sell_transaction');
}
else if (transaction status == 2) {
return $this->belongsTo('App\Car', 'id', 'buy_transaction');
}
}
}
I need to stick to that query structure because the query command is longer and I am joining and including other tables, I was hoping I could make the car() belongsTo relation conditional somehow.
I followed some similar situations like this but it didn't work for me.
Thank you.

Answer 1

The link you post has the answer
public function soldCar()
{
return $this->belongsTo('App\Car', 'id', 'sell_transaction');
}
public function boughtCar()
{
return $this->belongsTo('App\Car', 'id', 'buy_transaction');
}
public function scopeCar($query)
{
return $query->when($this->type === '1',function($q){
return $q->with('soldCar');
})
->when($this->type === '2',function($q){
return $q->with('boughtCar');
});
}
Edit: Also, this looks like a prime example for polymorfic relations as documented here https://laravel.com/docs/5.5/eloquent-relationships#polymorphic-relations