# Task 2.1 Implementation: User Data Model and DynamoDB Users Table

## Overview
This task implements the User data model and DynamoDB Users table with full CRUD operations.

## Files Created

### 1. User Model (`internal/models/user.go`)
- Defines the `User` struct with all required fields:
  - `ID` (user_id): Unique identifier (UUID)
  - `Email`: User's email address
  - `OAuthProvider`: OAuth provider name (e.g., "google", "github")
  - `OAuthID`: OAuth provider's user ID
  - `CreatedAt`: Timestamp when user was created
  - `UpdatedAt`: Timestamp when user was last updated

### 2. DynamoDB Client (`internal/storage/dynamodb.go`)
- `NewDynamoDBClient`: Creates a new DynamoDB client with optional endpoint override
- `CreateUsersTable`: Creates the Users table with proper schema
  - Partition Key: `user_id` (String)
  - Billing Mode: Pay-per-request (on-demand)
  - Waits for table to be active before returning

### 3. User Storage Layer (`internal/storage/user_storage.go`)
Implements all CRUD operations:
- `CreateUser`: Creates a new user with auto-generated UUID
- `GetUserByID`: Retrieves a user by their ID
- `GetUserByOAuth`: Retrieves a user by OAuth provider and ID
- `UpdateUser`: Updates an existing user (auto-updates UpdatedAt)
- `DeleteUser`: Deletes a user by their ID

### 4. Unit Tests (`internal/storage/user_storage_test.go`)
Comprehensive test coverage:
- `TestCreateUser`: Verifies user creation with all fields
- `TestGetUserByID`: Tests user retrieval by ID
- `TestGetUserByID_NotFound`: Tests error handling for non-existent users
- `TestGetUserByOAuth`: Tests user retrieval by OAuth credentials
- `TestGetUserByOAuth_NotFound`: Tests error handling for OAuth lookup
- `TestUpdateUser`: Verifies user updates and timestamp changes
- `TestDeleteUser`: Tests user deletion
- `TestCreateUser_MultipleUsers`: Verifies multiple users can be created with unique IDs

### 5. Database Initialization (`cmd/init-db/main.go`)
Standalone CLI tool to initialize DynamoDB tables:
```bash
go run cmd/init-db/main.go -endpoint http://localhost:8000
```

### 6. Table Initialization Helper (`cmd/server/init_tables.go`)
Helper function for the main server to initialize tables on startup.

## DynamoDB Schema

### Users Table
```
Table Name: Users
Partition Key: user_id (String)
Billing Mode: Pay-per-request

Attributes:
- user_id: String (UUID)
- email: String
- oauth_provider: String
- oauth_id: String
- created_at: Number (Unix timestamp in nanoseconds)
- updated_at: Number (Unix timestamp in nanoseconds)
```

## Dependencies Added
- `github.com/aws/aws-sdk-go-v2`: AWS SDK for Go v2
- `github.com/aws/aws-sdk-go-v2/config`: AWS configuration
- `github.com/aws/aws-sdk-go-v2/service/dynamodb`: DynamoDB service client
- `github.com/aws/aws-sdk-go-v2/feature/dynamodb/attributevalue`: DynamoDB attribute marshaling
- `github.com/google/uuid`: UUID generation

## Running Tests

### Prerequisites
1. Start DynamoDB Local:
   ```bash
   make db-start
   ```
   Or manually:
   ```bash
   docker-compose up -d dynamodb-local
   ```

2. Set environment variable (optional):
   ```bash
   export DYNAMODB_ENDPOINT=http://localhost:8000
   ```

### Run Tests
```bash
make test
# or
go test -v ./internal/storage/...
```

## Usage Example

```go
package main

import (
    "context"
    "log"
    
    "custom-start-page/internal/storage"
)

func main() {
    ctx := context.Background()
    
    // Create DynamoDB client
    db, err := storage.NewDynamoDBClient(ctx, "http://localhost:8000")
    if err != nil {
        log.Fatal(err)
    }
    
    // Create Users table
    if err := db.CreateUsersTable(ctx); err != nil {
        log.Fatal(err)
    }
    
    // Create user storage
    userStorage := storage.NewUserStorage(db)
    
    // Create a new user
    user, err := userStorage.CreateUser(ctx, "user@example.com", "google", "google123")
    if err != nil {
        log.Fatal(err)
    }
    
    log.Printf("Created user: %+v", user)
    
    // Retrieve user by ID
    retrievedUser, err := userStorage.GetUserByID(ctx, user.ID)
    if err != nil {
        log.Fatal(err)
    }
    
    log.Printf("Retrieved user: %+v", retrievedUser)
}
```

## Notes

### OAuth Lookup Performance
The `GetUserByOAuth` method currently uses a Scan operation since there's no GSI for `oauth_provider` + `oauth_id`. For production use with many users, consider adding a GSI:
- Partition Key: `oauth_provider`
- Sort Key: `oauth_id`

This would change the query from O(n) scan to O(1) lookup.

### Timestamp Storage
Timestamps are stored as Go `time.Time` which DynamoDB marshals as Unix timestamps in nanoseconds. This provides high precision for audit trails.

## Requirements Validated
- ✓ Requirement 1.1: User authentication foundation
- ✓ Requirement 1.3: OAuth user creation and retrieval
- ✓ Requirement 8.7: User data association

## Next Steps
Task 2.2 will implement the OAuth service using this User model to handle Google OAuth authentication.