# Phase 4: Basic Sales & POS - COMPLETE ✅

## Completion Date
December 16, 2024

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## Overview

Phase 4 implements basic POS sales operations including sale creation, inventory reservation/release, total calculation, invoice number generation, and sale management.

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## ✅ Completed Deliverables

### 1. Sales Service (`server/modules/sales/services/index.js`)
- ✅ `createSale()` - Create sale with items, reserve inventory, calculate totals
- ✅ `getSale()` - Get sale with items and associations
- ✅ `listSales()` - List sales with filters and pagination
- ✅ `cancelSale()` - Cancel DRAFT sales and release inventory
- ✅ `generateInvoiceNumber()` - Generate unique invoice numbers (INV-YYYYMMDD-XXXXX)
- ✅ `calculateItemTotals()` - Calculate line totals and VAT for items
- ✅ `calculateSaleTotals()` - Calculate sale subtotal, VAT, and total
- ✅ `getProductVatRate()` - Get VAT rate from tax category or product default

### 2. Sales Controllers (`server/modules/sales/controllers/index.js`)
- ✅ `createSale` - Create sale controller
- ✅ `getSale` - Get sale by ID controller
- ✅ `listSales` - List sales controller
- ✅ `cancelSale` - Cancel sale controller
- ✅ System logging integration
- ✅ Proper error handling

### 3. Sales Validations (`server/modules/sales/validations/index.js`)
- ✅ Validation rules for all endpoints
- ✅ Item validation (product_id, quantity, unit_price, variant_id)
- ✅ Sale type and status validation
- ✅ Date range validation for listing

### 4. Sales Routes (`server/modules/sales/routes/index.js`)
- ✅ RESTful endpoints with Swagger documentation
- ✅ All routes integrated into main routes
- ✅ Authentication required
- ✅ Role-based access where applicable

### 5. Database Schema Updates
- ✅ Added `user_id` field to Sale model (links to cashier)
- ✅ Added User association to Sale model
- ✅ Added user_id index for filtering

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## 📡 API Endpoints

### Create Sale
- **Endpoint:** `POST /api/sales`
- **Auth:** Required
- **Body:**
  ```json
  {
    "customer_id": 1,
    "sale_type": "POS",
    "items": [
      {
        "product_id": 1,
        "variant_id": 2,
        "quantity": 2,
        "unit_price": 1000
      }
    ]
  }
  ```
- **Returns:** Created sale with items and calculated totals

### Get Sale by ID
- **Endpoint:** `GET /api/sales/:id`
- **Auth:** Required
- **Returns:** Sale with items, customer, and cashier information

### List Sales
- **Endpoint:** `GET /api/sales?page=1&limit=10&status=DRAFT&customer_id=1&start_date=2024-01-01&end_date=2024-12-31&user_id=1`
- **Auth:** Required
- **Filters:**
  - `status` - Filter by status (DRAFT, PAID, CANCELLED)
  - `customer_id` - Filter by customer
  - `sale_type` - Filter by sale type (POS, INVOICE)
  - `start_date` - Filter by start date (ISO 8601)
  - `end_date` - Filter by end date (ISO 8601)
  - `user_id` - Filter by cashier
- **Returns:** Paginated sales list

### Cancel Sale
- **Endpoint:** `PUT /api/sales/:id/cancel`
- **Auth:** Required
- **Returns:** Cancelled sale
- **Note:** Only DRAFT sales can be cancelled

---

## 🔑 Key Features

### Sale Creation
- Creates sale with multiple items
- Generates unique invoice number (INV-YYYYMMDD-XXXXX)
- Calculates totals (subtotal, VAT, total)
- Reserves inventory on creation
- Links sale to cashier (user_id)
- Sets initial status to DRAFT

### Inventory Management
- **Reservation:** Inventory quantity reduced when sale created
- **Release:** Inventory quantity increased when sale cancelled
- **Movement Tracking:** All changes logged to inventory_movements
- **Validation:** Prevents negative inventory

### Total Calculation
- **Subtotal:** Sum of all item line totals
- **VAT:** Sum of all item VAT amounts
- **Total:** Subtotal + VAT
- **VAT Rate:** Retrieved from tax category or product default_vat_rate

### Invoice Numbers
- Format: `INV-YYYYMMDD-XXXXX`
- Sequential numbering per day
- Unique invoice numbers
- Example: `INV-20241216-00001`

### Sale Cancellation
- Only DRAFT sales can be cancelled
- Releases reserved inventory
- Updates sale status to CANCELLED
- Logs cancellation to system logs

### Filtering & Listing
- Filter by customer, status, sale type, date range, cashier
- Pagination support
- Ordered by creation date (newest first)

---

## 🔄 Integration Points

### With Phase 2 (Products)
- Uses Product and ProductVariant models
- Respects product track_inventory flag
- Uses product default_vat_rate or tax_category

### With Phase 3 (Inventory)
- Reserves inventory on sale creation
- Releases inventory on sale cancellation
- Creates inventory movements for audit trail
- Prevents negative inventory

### With Phase 1 (Users)
- Links sales to cashier via user_id
- Tracks who created each sale

---

## 📊 Database Schema

### Sale Table
- `id` - Primary key
- `invoice_no` - Unique invoice number (INV-YYYYMMDD-XXXXX)
- `customer_id` - Foreign key to customers (nullable)
- `user_id` - Foreign key to users (cashier, nullable)
- `sale_type` - ENUM (POS, INVOICE)
- `shift_id` - Foreign key to shifts (nullable)
- `status` - ENUM (DRAFT, PAID, CANCELLED)
- `subtotal` - DECIMAL(12,2)
- `vat` - DECIMAL(12,2)
- `total` - DECIMAL(12,2)

### SaleItem Table
- `id` - Primary key
- `sale_id` - Foreign key to sales
- `product_id` - Foreign key to products
- `variant_id` - Foreign key to product_variants (nullable)
- `inventory_item_id` - Foreign key to inventory_items (nullable)
- `quantity` - DECIMAL(12,3)
- `unit_price` - DECIMAL(12,2)
- `vat_rate` - DECIMAL(5,2)

---

## 📝 Business Rules

1. **Sale Creation:**
   - Must have at least one item
   - Unit price required (or retrieved from pricing)
   - Inventory checked before creation
   - Inventory reserved on creation
   - Invoice number auto-generated
   - Initial status is DRAFT

2. **Sale Cancellation:**
   - Only DRAFT sales can be cancelled
   - Inventory released on cancellation
   - Status changed to CANCELLED
   - Cannot cancel PAID sales

3. **Inventory Reservation:**
   - Quantity reduced when sale created
   - Movement recorded with reason 'SALE'
   - Reference ID set to sale ID
   - Prevents negative inventory

4. **Total Calculation:**
   - Line total = quantity * unit_price
   - VAT amount = line_total * vat_rate / 100
   - Subtotal = sum of line totals
   - Total VAT = sum of VAT amounts
   - Total = subtotal + VAT

5. **VAT Rate:**
   - Priority: Tax Category > Product default_vat_rate > 0
   - VAT rate stored per item for historical accuracy

---

## ✅ Acceptance Criteria Met

- ✅ Can create sale with items
- ✅ Totals calculated correctly
- ✅ Inventory reserved on sale creation
- ✅ Sale can be cancelled (inventory released)
- ✅ Invoice numbers unique
- ✅ Sales linked to cashier
- ✅ Sale listing with filters working
- ✅ Multiple items in sale supported
- ✅ Transaction support for data integrity

---

## 🚀 Next Steps

Phase 5: Payments Integration
- Implement payment processing
- Integrate Paystack API
- Cash payment processing
- Mobile money payment (mock/stub)
- Payment status tracking
- Link payments to sales
- Auto-update sale status when fully paid

---

**Phase 4 Status: COMPLETE ✅**

