# Requirement: Expense Account for Deductions

## Summary

**Payment deductions require the Expense Account field to be set.**

This is a mandatory requirement enforced by validation.

## Why This Requirement?

### Technical Reason

When creating a payment with deductions, the journal entry structure is:

```
Expense Account (Debit)  ← Must be specified
Tax Accounts (Credit)    ← Deductions
Bank Account (Credit)    ← Net payment
```

Without the Expense Account, the system cannot determine which account should receive the debit entry.

### Accounting Reason

The Expense Account represents:
- The full cost/expense being incurred
- The account that should be debited for the total amount
- The proper classification of the expense

Example:
- Telepon & Internet expense: Rp 2,000,000
- Less: PPh 21 withheld: Rp 100,000
- Less: PPh 29 withheld: Rp 50,000
- Net payment to vendor: Rp 1,850,000

The Expense Account (Telepon & Internet) gets the full Rp 2,000,000 debit.

## Validation

The module enforces this requirement with a validation constraint:

```python
@api.constrains('amount', 'amount_substract', 'expense_account_id', 'deduction_line_ids')
def _check_amount_substract(self):
    for payment in self:
        # ... other validations ...
        
        # Require expense account when using deductions
        if payment.deduction_line_ids and not payment.expense_account_id:
            raise ValidationError(_(
                "Expense Account is required when using payment deductions.\n\n"
                "Please set the Expense Account field before adding deductions."
            ))
```

### When Validation Triggers

The validation error appears when:
1. You add deduction lines
2. But haven't set the Expense Account field
3. And try to save or post the payment

### Error Message

```
Expense Account is required when using payment deductions.

Please set the Expense Account field before adding deductions.
```

## How to Use

### Correct Workflow

```
1. Create Payment
   ↓
2. Set Expense Account ✅ (e.g., "Telepon & Internet")
   ↓
3. Add Deductions (e.g., PPh 21, PPh 29)
   ↓
4. Post Payment ✅
```

### Incorrect Workflow (Will Fail)

```
1. Create Payment
   ↓
2. Add Deductions ❌
   ↓
3. Try to Post ❌
   ↓
Error: "Expense Account is required..."
```

## Alternative: Without Deductions

If you don't need deductions, you can use the standard payment flow:

```
1. Create Payment
   ↓
2. Don't set Expense Account (optional)
   ↓
3. Don't add Deductions
   ↓
4. Post Payment ✅
```

In this case, the system uses the standard Accounts Payable account.

## Comparison

### With Expense Account + Deductions

**Journal Entry:**
```
Expense Account          Debit: 2,000,000
PPh 21                   Credit: 100,000
PPh 29                   Credit: 50,000
Bank                     Credit: 1,850,000
```

**Use Case:** Direct expense recording with tax withholding

### Without Expense Account (Standard)

**Journal Entry:**
```
Accounts Payable         Debit: 2,000,000
Bank                     Credit: 2,000,000
```

**Use Case:** Standard vendor payment without deductions

## Benefits of This Requirement

### 1. Clear Accounting

Forces users to specify exactly which expense account should be used, ensuring:
- Proper expense classification
- Accurate financial reporting
- Clear audit trail

### 2. Prevents Errors

Prevents common mistakes like:
- Missing expense account
- Unclear journal entries
- Unbalanced entries

### 3. Consistent Behavior

Ensures all payments with deductions follow the same pattern:
- Always have an expense account
- Always have proper journal entries
- Always have correct tax treatment

## Configuration

### Required Accounts

Before using deductions, set up:

#### 1. Expense Accounts
```
Chart of Accounts > Create Account
- Name: Telepon & Internet
- Code: 611505
- Type: Expenses
```

#### 2. Tax Payable Accounts
```
Chart of Accounts > Create Account
- Name: PPh 21
- Code: 217101
- Type: Current Liabilities
```

### Account Selection

When creating a payment with deductions:

**Expense Account:** Choose from expense accounts
- ✅ Telepon & Internet
- ✅ Office Supplies
- ✅ Professional Fees
- ❌ Accounts Payable (not allowed)

**Deduction Accounts:** Choose from tax/liability accounts
- ✅ PPh 21 (Tax Payable)
- ✅ PPh 23 (Tax Payable)
- ✅ PPh 29 (Tax Payable)
- ❌ Accounts Payable (not allowed)
- ❌ Bank accounts (not allowed)

## User Training

### Key Points to Teach Users

1. **Always set Expense Account first** when using deductions
2. **Choose the right expense account** for the type of expense
3. **Use tax accounts** for deductions, not payable accounts
4. **Verify amounts** before posting

### Common Questions

**Q: Why can't I just use Accounts Payable?**

A: When using deductions, you're recording the expense directly. Accounts Payable is for tracking vendor balances, not expenses.

**Q: What if I forget to set Expense Account?**

A: You'll get a validation error. Just set the Expense Account field and try again.

**Q: Can I change the Expense Account after adding deductions?**

A: Yes, as long as the payment hasn't been posted yet.

## Technical Details

### Module Integration

This requirement is part of the `vendor_payment_diff_amount` module v2.0.0.

It works with:
- `vendor_batch_payment_merge` (provides expense_account_id field)
- Standard Odoo accounting (account.payment)

### Field Definition

The `expense_account_id` field is defined in `vendor_batch_payment_merge`:

```python
expense_account_id = fields.Many2one(
    'account.account',
    string='Expense Account',
    domain="[('account_type', 'not in', ('asset_receivable', 'liability_payable'))]",
    help="Account used for expense instead of the default payable/receivable account"
)
```

### Validation Logic

The validation is in `vendor_payment_diff_amount`:

```python
if payment.deduction_line_ids and not payment.expense_account_id:
    raise ValidationError("Expense Account is required...")
```

## Summary

✅ **DO:** Set Expense Account before adding deductions
✅ **DO:** Use expense accounts for Expense Account field
✅ **DO:** Use tax accounts for deduction accounts
❌ **DON'T:** Try to add deductions without Expense Account
❌ **DON'T:** Use Accounts Payable for deductions

---

**Remember: Expense Account is REQUIRED when using deductions!**

For more information, see:
- USER_GUIDE.md - Step-by-step instructions
- README.md - Module overview
- FINAL_FIX.md - Technical details