Add initial Code

app/Models/Invoice.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use App\Models\Customer;
+use App\Models\LineItem;
+
+class Invoice extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'nr',
+        'invoice_date',
+        'due_date',
+        'service_start_date',
+        'service_end_date',
+        'is_recurring',
+        'is_partial_service',
+        'customer_id',
+        'billing_data',
+        'payment_status',
+        'total_amount',
+        'title',
+        'text',
+        'items'
+    ];
+
+    protected $casts = [
+        'billing_data' => 'array', // JSON to  Array
+    ];
+
+    public function customer()
+    {
+        return $this->belongsTo(Customer::class);
+    }
+
+    public function items()
+    {
+        return $this->hasMany(LineItem::class);
+    }
+
+    /**
+     * Get the payment term for the invoice.
+     */
+    public function getPaymentTermsAttribute()
+    {
+        return $this->customer->paymentTerms;
+    }
+}