Add all files

app/Models/InvoiceTransaction.php

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+<?php
+
+namespace App\Models;
+
+use App\Classes\Price;
+use App\Observers\InvoiceTransactionObserver;
+use Illuminate\Database\Eloquent\Attributes\ObservedBy;
+use Illuminate\Database\Eloquent\Casts\Attribute;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+#[ObservedBy([InvoiceTransactionObserver::class])]
+class InvoiceTransaction extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'invoice_id',
+        'gateway_id',
+        'amount',
+        'fee',
+        'transaction_id',
+    ];
+
+    public function invoice()
+    {
+        return $this->belongsTo(Invoice::class);
+    }
+
+    public function gateway()
+    {
+        return $this->belongsTo(Gateway::class);
+    }
+
+    /**
+     * Formatted remaining amount of the invoice.
+     */
+    public function formattedFee(): Attribute
+    {
+        return Attribute::make(
+            get: fn () => new Price(['price' => $this->fee, 'currency' => $this->invoice->currency])
+        );
+    }
+
+    /**
+     * Formatted remaining amount of the invoice.
+     */
+    public function formattedAmount(): Attribute
+    {
+        return Attribute::make(
+            get: fn () => new Price(['price' => $this->amount, 'currency' => $this->invoice->currency])
+        );
+    }
+}