invoiceninja/app/Models/InvoiceItem.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\SoftDeletes;
use Laracasts\Presenter\PresentableTrait;

/**
 * Class InvoiceItem.
 */
class InvoiceItem extends EntityModel
{
    use PresentableTrait;
    use SoftDeletes;

    /**
     * @var string
     */
    protected $presenter = 'App\Ninja\Presenters\InvoiceItemPresenter';

    /**
     * @var array
     */
    protected $dates = ['deleted_at'];

    /**
     * @return mixed
     */
    public function getEntityType()
    {
        return ENTITY_INVOICE_ITEM;
    }

    /**
     * @var array
     */
    protected $fillable = [
        'tax_name1',
        'tax_rate1',
        'tax_name2',
        'tax_rate2',
    ];

    /**
     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
     */
    public function invoice()
    {
        return $this->belongsTo('App\Models\Invoice');
    }

    /**
     * @return mixed
     */
    public function user()
    {
        return $this->belongsTo('App\Models\User')->withTrashed();
    }

    /**
     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
     */
    public function product()
    {
        return $this->belongsTo('App\Models\Product');
    }

    /**
     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
     */
    public function account()
    {
        return $this->belongsTo('App\Models\Account');
    }

    public function amount()
    {
        $amount = $this->cost * $this->qty;
        $preTaxAmount = $amount;

        if ($this->tax_rate1) {
            $amount += $preTaxAmount * $this->tax_rate1 / 100;
        }

        if ($this->tax_rate2) {
            $amount += $preTaxAmount * $this->tax_rate2 / 100;
        }

        return $amount;
    }
}