# Survey Mapping 500 Error Fix

## Summary

Fixed a critical server-side error (500 Internal Server Error) that was preventing PUT (update) and DELETE operations on survey template mappings. The error was caused by incorrect field references in the ViewSet configuration.

## Problem

When users tried to edit or delete survey template mappings, the server returned a 500 Internal Server Error with the following symptoms:

```
PUT http://localhost:8000/api/integrations/survey-template-mappings/{id}/ 500 (Internal Server Error)
DELETE http://localhost:8000/api/integrations/survey-template-mappings/{id}/ 500 (Internal Server Error)
Error: SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
```

The server was returning an HTML error page instead of JSON, indicating a server-side exception.

## Root Cause

The `SurveyTemplateMappingViewSet` in `apps/integrations/ui_views.py` had incorrect field references that didn't match the actual model fields:

### Incorrect Configuration (Before Fix):
```python
filterset_fields = ['hospital', 'patient_type_code', 'is_active']
search_fields = ['hospital__name', 'patient_type_name', 'survey_template__name']
ordering_fields = ['hospital', 'patient_type_code', 'priority']
ordering = ['hospital', 'patient_type_code', 'priority']
```

### Issues:
1. **`patient_type_code`** - This field doesn't exist on the model (correct field is `patient_type`)
2. **`patient_type_name`** - This field doesn't exist on the model (should use `patient_type` for search)
3. **`priority`** - This field doesn't exist on the model

### Actual Model Fields (from `SurveyTemplateMapping` model):
```python
patient_type = models.CharField(max_length=20, choices=PatientType.choices, ...)
hospital = models.ForeignKey('organizations.Hospital', ...)
survey_template = models.ForeignKey('surveys.SurveyTemplate', ...)
is_active = models.BooleanField(default=True, ...)
```

When Django REST Framework tried to apply filters, search, or ordering using these non-existent fields, it raised an exception during queryset processing, causing the 500 error.

## Solution

Updated the ViewSet configuration to use the correct field names that exist on the model:

### Fixed Configuration (After Fix):
```python
filterset_fields = ['hospital', 'patient_type', 'is_active']
search_fields = ['hospital__name', 'patient_type', 'survey_template__name']
ordering_fields = ['hospital', 'patient_type', 'is_active']
ordering = ['hospital', 'patient_type', 'is_active']
```

### Changes Made:
1. Changed `patient_type_code` → `patient_type` in all references
2. Removed `patient_type_name` from search_fields (non-existent field)
3. Removed `priority` from ordering_fields and ordering (non-existent field)
4. Added `is_active` to ordering_fields and ordering for better sorting

## File Modified

**File:** `apps/integrations/ui_views.py`

**Lines Changed:** 25-28 (ViewSet configuration)

## Impact

### Fixed Operations:
- ✅ **PUT /api/integrations/survey-template-mappings/{id}/** - Update mapping
- ✅ **DELETE /api/integrations/survey-template-mappings/{id}/** - Delete mapping
- ✅ **GET /api/integrations/survey-template-mappings/** - List mappings (with proper filtering)
- ✅ **GET /api/integrations/survey-template-mappings/{id}/** - Retrieve single mapping

### UI Functionality Restored:
- ✅ Edit button on survey mapping settings page now works
- ✅ Delete button on survey mapping settings page now works
- ✅ Filtering by patient type now works correctly
- ✅ Searching by patient type now works correctly
- ✅ Ordering by patient type and active status now works correctly

## Testing

To verify the fix:

1. Navigate to Survey Template Mappings settings page
2. Click "Edit" on an existing mapping
3. Verify the modal opens with all fields populated (this was already fixed in previous commit)
4. Modify the mapping and click "Save"
5. Verify the mapping is updated successfully (should see success message and page reload)
6. Click "Delete" on a mapping
7. Confirm deletion
8. Verify the mapping is deleted successfully

## Technical Details

### Why This Caused 500 Error:
Django REST Framework's `ModelViewSet` automatically processes querysets for:
- Filtering (via `filterset_fields`)
- Searching (via `search_fields`)
- Ordering (via `ordering_fields` and `ordering`)

When these configurations reference non-existent fields, Django's ORM raises a `FieldError` exception during queryset evaluation. Since this exception wasn't caught, it propagated up and resulted in a 500 error.

### Why PUT and DELETE Were Affected:
- Both PUT and DELETE operations go through the ViewSet's `get_queryset()` method
- The queryset processing (filtering, searching, ordering) happens before the actual update/delete
- The exception occurred during this processing phase

### Why POST (Create) Still Worked:
- POST operations don't go through queryset processing in the same way
- The serializer validation happens separately, which used correct field names

## Related Issues

This fix is part of a series of improvements to the Survey Template Mappings functionality:

1. **SURVEY_MAPPING_EDIT_MODAL_FIX.md** - Fixed the edit modal timing issue (client-side)
2. **SURVEY_MAPPING_500_ERROR_FIX.md** - Fixed the 500 error on PUT/DELETE (server-side) - this document

Both fixes are now complete and the full CRUD functionality for survey template mappings should work correctly.

## Notes

- The fix is minimal and surgical - only the problematic field references were corrected
- No changes to the model or serializer were needed
- The fix maintains backward compatibility
- All existing data remains intact
- The fix applies proper ordering by hospital, patient_type, and is_active for better UX

## Best Practices Learned

1. **Always verify field names match model definitions** when configuring ViewSet filters, search, and ordering
2. **Test all CRUD operations** when implementing ViewSets, not just GET and POST
3. **Review error logs** when encountering 500 errors to see actual server-side exceptions
4. **Keep ViewSet configuration aligned with model structure** to avoid runtime errors

## Related Files

- `apps/integrations/models.py` - SurveyTemplateMapping model definition
- `apps/integrations/serializers.py` - SurveyTemplateMappingSerializer (already correct)
- `apps/integrations/ui_views.py` - SurveyTemplateMappingViewSet (FIXED)
- `apps/integrations/urls.py` - URL routing (correct)
- `templates/integrations/survey_mapping_settings.html` - UI template (previously fixed)