# Fix Summary: "Missing required account on accountable line" Error

## Problem

When creating a vendor payment with deductions, the system threw the error:
```
The operation cannot be completed: Missing required account on accountable line.
```

This occurred because Odoo validates that lines with payable/receivable accounts must have a `partner_id` set.

## Root Cause

The issue had two parts:

1. **Deduction lines were incorrectly getting `partner_id`**: All deduction lines were being created with `partner_id`, but deduction accounts (like tax accounts) are typically NOT payable/receivable accounts and should NOT have a partner.

2. **Odoo's validation**: When a line has a payable/receivable account type, Odoo requires the `partner_id` field. When a line has other account types (expense, liability, etc.), the `partner_id` should be optional or omitted.

## Solution

Modified the `_prepare_move_line_default_vals` method to:

1. **Ensure counterpart line has partner**: The payable/expense line (counterpart) always gets `partner_id` set
2. **Conditional partner on deduction lines**: Only add `partner_id` to deduction lines if the account type requires it

### Code Change

**Before:**
```python
deduction_line = {
    'name': deduction_line_name,
    'date_maturity': self.date,
    'amount_currency': -deduction.amount_substract,
    'currency_id': self.currency_id.id,
    'debit': 0.0,
    'credit': deduction_balance,
    'partner_id': self.partner_id.id,  # ❌ Always added
    'account_id': deduction.substract_account_id.id,
}
```

**After:**
```python
deduction_line = {
    'name': deduction_line_name,
    'date_maturity': self.date,
    'amount_currency': -deduction.amount_substract,
    'currency_id': self.currency_id.id,
    'debit': 0.0,
    'credit': deduction_balance,
    'account_id': deduction.substract_account_id.id,
}

# Only add partner_id if the account requires it
if deduction.substract_account_id.account_type in ('asset_receivable', 'liability_payable'):
    deduction_line['partner_id'] = self.partner_id.id  # ✅ Conditionally added
```

## Account Types

### Accounts that REQUIRE partner_id:
- `asset_receivable` (Customer accounts)
- `liability_payable` (Vendor accounts)

### Accounts that DON'T need partner_id:
- `liability_current` (Tax payable accounts like PPh 21, PPh 29)
- `expense` (Expense accounts)
- `income` (Income accounts)
- `asset_cash` (Bank accounts)
- All other account types

## Result

Now the journal entry is created correctly:

### Example: Payment Rp 2,000,000 with PPh 21 (Rp 100,000) and PPh 29 (Rp 50,000)

```
Account                              | Debit           | Credit          | Partner
-------------------------------------|-----------------|-----------------|------------------
Accounts Payable                     | Rp 2,000,000.00 |                 | PT Telkom ✅
PPh 21 (Tax Payable)                 |                 | Rp 100,000.00   | (none) ✅
PPh 29 (Tax Payable)                 |                 | Rp 50,000.00    | (none) ✅
Bank Account                         |                 | Rp 1,850,000.00 | (none) ✅
-------------------------------------|-----------------|-----------------|------------------
TOTAL                                | Rp 2,000,000.00 | Rp 2,000,000.00 |
```

### Key Points:
- ✅ Payable account has partner (required)
- ✅ Tax accounts don't have partner (correct)
- ✅ Bank account doesn't have partner (correct)
- ✅ Entry is balanced
- ✅ No validation errors

## Testing

To test the fix:

1. **Create a payment without expense account:**
   ```
   - Partner: Any vendor
   - Amount: 1000
   - Deduction: PPh 21 - 100 (use a tax account)
   - Result: Should post successfully
   ```

2. **Create a payment with expense account:**
   ```
   - Partner: Any vendor
   - Amount: 2000
   - Expense Account: Telepon & Internet
   - Deduction 1: PPh 21 - 100
   - Deduction 2: PPh 29 - 50
   - Result: Should post successfully
   ```

3. **Verify journal entries:**
   - Check that payable/expense line has partner
   - Check that tax lines don't have partner
   - Check that entry is balanced

## Important Notes

### Deduction Account Selection

When adding deductions, make sure to use the correct account types:

✅ **Correct accounts for deductions:**
- Tax payable accounts (PPh 21, PPh 23, PPh 29, etc.)
- Expense accounts (if recording as expense)
- Liability accounts (for other withholdings)

❌ **Don't use these for deductions:**
- Accounts Payable (vendor accounts)
- Accounts Receivable (customer accounts)

### Why This Matters

Using payable/receivable accounts for deductions would create confusion:
- It would require a partner on the deduction line
- It would mix vendor payables with tax payables
- It would complicate reconciliation
- It's not the correct accounting treatment

## Files Modified

1. **`models/account_payment.py`**
   - Added conditional `partner_id` logic for deduction lines
   - Ensured counterpart line always has `partner_id`

2. **`SCENARIOS.md`**
   - Updated validation rules
   - Added explanation about partner_id handling
   - Updated troubleshooting section

3. **`FIX_SUMMARY.md`** (this file)
   - Documented the fix and reasoning

## Version

This fix is included in version 2.0.0 of the module.

## Related Issues

- "Missing required account on accountable line" error
- Partner validation on journal entry lines
- Deduction account configuration

## Credits

Fixed based on user feedback and testing with real-world scenarios.