🚀 Implement Expense Service - Core Backend Component

[Devasy]

🎯 Overview

The Expense Service is a critical missing component of the Splitwiser backend. This service will handle expense management, splitting algorithms, and settlement calculations.

📋 Requirements

Core Features to Implement:

• Expense CRUD Operations

  • Create, read, update, delete expenses within groups
  • Support for different splitting methods (equal, percentage, custom amounts)
  • Expense validation and business logic

• Smart Settlement System

  • Implement directed graph algorithm for debt minimization
  • Calculate optimized settlements to reduce transaction count
  • Support for manual settlement recording

• Friends Balance Aggregation

  • Cross-group balance calculations
  • Real-time balance updates
  • User balance summaries

• File Attachment Support

  • Receipt upload functionality
  • File storage integration (S3-compatible)
  • Attachment management

API Endpoints to Implement:

POST   /groups/{group_id}/expenses
GET    /groups/{group_id}/expenses
GET    /groups/{group_id}/expenses/{expense_id}
PATCH  /groups/{group_id}/expenses/{expense_id}
DELETE /groups/{group_id}/expenses/{expense_id}
POST   /groups/{group_id}/expenses/{expense_id}/attachments
GET    /groups/{group_id}/expenses/{expense_id}/attachments/{key}
POST   /groups/{group_id}/settlements
GET    /groups/{group_id}/settlements
PATCH  /groups/{group_id}/settlements/{settlement_id}
DELETE /groups/{group_id}/settlements/{settlement_id}
POST   /groups/{group_id}/settlements/optimize
GET    /users/me/friends-balance
GET    /users/me/balance-summary
GET    /groups/{group_id}/users/{user_id}/balance

🔗 Dependencies

• Integrates with existing Group Service
• Requires authentication via Auth Service
• Uses MongoDB collections: expenses, settlements

📚 Documentation

• Expense Service API Spec
• Database Schema
• Micro-plan

🏗️ Implementation Tasks

1. Create backend/app/expenses/ module
2. Implement expense models and schemas
3. Create expense service with business logic
4. Build REST API routes
5. Add comprehensive test coverage
6. Integrate with main application

🎯 Priority

High Priority - This is a core feature required for MVP functionality.

🏷️ Labels

enhancement, backend, high-priority, expense-management