Laravel - Sums, Complex Relationships and Models Issue - laravel

Question

I've got a customer index table that I would like to be able to show the current balance a customer has by comparing the payments made to bills against their total, and then sum up all of the remaining amounts.
At the moment my customer model looks like this (specifically for this question):
public function billToShipments()
{
return $this->hasMany(Shipment::class, 'bill_to');
}
and then my shipment model looks like this (in relation to the payment distributions):
public function paymentDistributions(){
return $this->hasMany('App\Payments_Distribution', 'shipment_id','pro_number');
}
These are the necessary fields related to this specific question:
Under Payment Distributions
-pro_number
-amount (of distribution)
Under Shipments
-balance_due
-bill_to (which is the customer id)
What I'd like to be able to do is get the sum of all balance_due's of bills that have less payment_distributions than the balance due for the customer.
For example, in the shipments (pretend under one customer):
SHIPMENT ID | Balance_Due
1234 | 10.00
1235 | 20.00
1236 | 30.00
and in the payment_distributions table:
PRO_NUMBER | AMOUNT
1234 | 2.00
1234 | 4.00
1235 | 20.00
1236 | 28.00
On the customer I'd like to say that they have a $6.00 balance (because 10.00 minus (2.00 plus 4.00) equals 4.00 for shipment # 1234, $20.00 pays off shipment #1235 completely and 2.00 remains for shipment #1236.
Again, I'd like to use the balance in a table of customers (under an #foreach statement) to give a balance sheet.
---- Updated for Jonas ----
This is how at the moment I grab remaining sums on a one record view for a customer, I pass the following to the view from the controller:
$invoicesOpen = Shipment
::whereRaw('balance > (SELECT IFNULL(SUM(payments_distributions.amount),0) FROM payments_distributions WHERE payments_distributions.shipment_id = pro_number)')
->where('bill_to','=',$customer->id)
->whereNotIn('shipment_billing_status', [2,3])
->orderBy('created_at','desc')->get();
At this point I can see that the customer has $1172.60 remaining, but through Jonas' suggestion, I somehow get -$5477.90
Now there is a closed invoice total that I can get which is currently at $5240.30. This last number is here solely to show that I'm not sure how Jonas' total is calculated.

Answer 1

Add this to your Customer model:
public function getBalanceAttribute() {
$toPay = $this->sum($this->billToShipments->pluck('balance_due'));
$payments = $this->billToShipments->pluck('paymentDistributions')->collapse();
$paid = $this->sum($payments->pluck('amount'));
return bcsub($toPay, $paid, 2);
}
protected function sum($values) {
return $values->reduce(function($carry, $item) {
return bcadd($carry, $item, 2);
}, '0');
}
Then use it like this:
$customers = Customer::with('billToShipments.paymentDistributions')->get();
foreach($customers as $customer) {
// $customer->balance
}