# Task 11 Implementation Summary: Missed Payments Service and Detail Page

## Overview
Successfully implemented the missed payments service and detail page functionality for the reports system enhancement project.

## Components Implemented

### 1. Missed Payments Service (`reports/missed_payments_service.py`)
Created a centralized service for tracking and reporting missed payments with the following methods:

#### `get_clients_with_missed_payments(branch_id=None)`
- Returns list of clients who have loans with missed payments
- Groups loans by client
- Calculates total outstanding and missed payment counts per client
- Sorts by missed payment count (descending)
- Supports optional branch filtering

#### `get_missed_payments_for_client(client_id)`
- Returns all missed payments for a specific client
- Filters for loans where `due_date < current_date AND outstanding_amount > 0`
- Excludes rolled-over and soft-deleted loans
- Sorts by due_date in descending order (most recent first)
- Calculates days overdue for each loan

#### `calculate_missed_payment_count(client)`
- Calculates the number of missed payments for a specific client
- Uses the same criteria as other methods for consistency

#### `get_missed_payment_details(loan)`
- Provides granular details about missed payment periods for a loan
- Returns structured data for display in reports

### 2. Missed Payments Detail View (`reports/views.py`)
Added new view function `missed_payments_detail(request, client_id)`:
- Displays all missed payments for a specific client
- Shows summary metrics (total count, total outstanding, most overdue)
- Provides detailed information for each missed payment
- Includes links to loan details and client contact information

### 3. URL Configuration (`reports/urls.py`)
Added new URL pattern:
```python
path('missed-payments/client/<uuid:client_id>/', views.missed_payments_detail, name='missed_payments_detail')
```

### 4. Detail Page Template (`templates/reports/missed_payments_detail.html`)
Created comprehensive template with:
- Client header with avatar and contact information
- Summary metrics cards (total missed, total outstanding, most overdue)
- Detailed list of all missed payments with:
  - Loan number and product
  - Due date and days overdue
  - Amount due and payment status
  - Color-coded severity indicators (critical, warning, normal)
  - Action buttons (View Loan, Call Client)
- Empty state for clients with no missed payments
- Responsive design with modern styling

### 5. Updated Missed Payments Report Template
Enhanced `templates/reports/missed_payments_report.html`:
- Added "View All Missed Payments" button for each client
- Links to the new detail page
- Improved layout to accommodate new button
- Maintained existing functionality for daily/weekly/monthly categorization

### 6. Updated Missed Payments Report View
Modified `missed_payments_report` view in `reports/views.py`:
- Added `borrower_id` to loan_data for detail page links
- Maintained backward compatibility with existing functionality
- Added documentation referencing requirements

## Property-Based Tests

### Test File: `reports/test_missed_payments_properties.py`
Created comprehensive property-based tests using Hypothesis:

#### Property 26: Missed Payment Detail Retrieval
- **Validates**: Requirements 10.2
- **Test**: `test_missed_payment_detail_includes_all_overdue_loans`
- **Property**: For any client, clicking "View All Missed Payments" should display all loans where `due_date < current_date AND outstanding_amount > 0`
- **Status**: ✅ PASSED

#### Property 27: Missed Payment Definition
- **Validates**: Requirements 10.4
- **Test**: `test_missed_payment_definition_correctness`
- **Property**: A loan has a missed payment IFF `due_date < today AND outstanding_amount > 0`
- **Status**: ✅ PASSED

#### Property 28: Missed Payments Sort Order
- **Validates**: Requirements 10.5
- **Test**: `test_missed_payments_sorted_by_due_date_descending`
- **Property**: For any list of missed payments, records should be ordered by due_date in descending order (most recent first)
- **Status**: ✅ PASSED

### Manual Verification Test: `test_missed_payments_simple.py`
Created standalone test to verify service functionality:
- ✅ Correctly identifies overdue loans with outstanding balance
- ✅ Excludes current loans (not yet due)
- ✅ Calculates missed payment count accurately
- ✅ Includes clients with missed payments in results
- ✅ Sorts results by due date descending

## Requirements Validated

### Requirement 10.1
✅ "WHEN viewing the missed payments report THEN the System SHALL display a button for each client labeled 'View All Missed Payments'"
- Implemented button in template for each loan entry

### Requirement 10.2
✅ "WHEN a user clicks the 'View All Missed Payments' button THEN the System SHALL navigate to a detail page showing all missed payments for that client"
- Implemented detail view and URL routing
- Validated by Property 26

### Requirement 10.3
✅ "WHEN displaying the missed payments detail page THEN the System SHALL show loan number, due date, amount due, and days overdue for each missed payment"
- All fields displayed in detail template
- Additional fields included (product name, total amount, amount paid, status)

### Requirement 10.4
✅ "WHEN calculating missed payments THEN the System SHALL identify payments where due date has passed and outstanding amount is greater than zero"
- Implemented in service logic
- Validated by Property 27

### Requirement 10.5
✅ "WHEN ordering missed payments THEN the System SHALL sort by due date in descending order with most recent first"
- Implemented in `get_missed_payments_for_client` method
- Validated by Property 28

## Key Features

### Correctness Properties
1. **Date-based filtering**: Only includes loans where `due_date < current_date`
2. **Outstanding balance check**: Only includes loans with `outstanding_amount > 0`
3. **Exclusion logic**: Properly excludes rolled-over and soft-deleted loans
4. **Sort order**: Consistently sorts by due_date descending
5. **Data consistency**: All methods use the same criteria for identifying missed payments

### User Experience
1. **Easy navigation**: One-click access from main report to client detail
2. **Visual hierarchy**: Color-coded severity indicators (critical/warning/normal)
3. **Comprehensive information**: All relevant loan and payment details displayed
4. **Action buttons**: Quick access to loan details and client contact
5. **Responsive design**: Works well on different screen sizes

### Performance Considerations
1. **Efficient queries**: Uses `select_related` for foreign keys
2. **Filtered at database level**: Excludes deleted/rolled-over loans in query
3. **Calculated properties**: Uses loan model's `outstanding_amount` property
4. **Minimal data transfer**: Only fetches necessary fields

## Testing Results

### Manual Testing
- ✅ Service correctly identifies missed payments
- ✅ Detail page displays all required information
- ✅ Navigation between reports works correctly
- ✅ Buttons and links function properly
- ✅ Data accuracy verified with test scenarios

### Property-Based Testing
- ✅ All 3 properties pass with 100 examples each
- ✅ Edge cases handled correctly (empty lists, boundary dates)
- ✅ Consistent behavior across random inputs

## Files Created/Modified

### Created:
1. `reports/missed_payments_service.py` - Service implementation
2. `templates/reports/missed_payments_detail.html` - Detail page template
3. `reports/test_missed_payments_properties.py` - Property-based tests
4. `test_missed_payments_simple.py` - Manual verification test
5. `TASK_11_IMPLEMENTATION_SUMMARY.md` - This document

### Modified:
1. `reports/views.py` - Added detail view, updated main report view
2. `reports/urls.py` - Added detail page URL pattern
3. `templates/reports/missed_payments_report.html` - Added "View All" buttons
4. `.kiro/specs/reports-system-enhancement/tasks.md` - Updated task status

## Conclusion

Task 11 has been successfully completed with all requirements met and validated through property-based testing. The implementation provides a robust, user-friendly solution for tracking and managing missed payments, with strong correctness guarantees and comprehensive test coverage.

The service follows the established patterns in the codebase, integrates seamlessly with existing functionality, and provides a solid foundation for future enhancements to the missed payments tracking system.
