Understanding the DAO Design Pattern for Multi-Table Operations

What is the DAO design pattern, and how can it be effectively utilized for managing multiple tables in a database?

// Example DAO class for handling multiple tables
public class UserDAO {
    private Connection connection;
    
    public UserDAO(Connection connection) {
        this.connection = connection;
    }
    
    public User getUserById(int id) {
        // SQL query to retrieve user by ID from the 'users' table
        String query = "SELECT * FROM users WHERE id = ?";
    }
    
    public List<User> getUsersFromDifferentTables() {
        // Logic to retrieve users and related data from multiple tables
    }
}

The Data Access Object (DAO) design pattern is a structural pattern that provides an abstraction layer for accessing data from a data source. It is designed to separate the data access logic from the business logic, making it easier to maintain and test the code. This is particularly beneficial when dealing with multiple database tables, as it allows you to encapsulate the logic for each table in its own DAO. Below, we explore how to implement the DAO pattern for managing operations across multiple tables.

// Example of using multiple DAOs within a service class
public class UserService {
    private UserDAO userDAO;
    private OrderDAO orderDAO;
    
    public UserService(UserDAO userDAO, OrderDAO orderDAO) {
        this.userDAO = userDAO;
        this.orderDAO = orderDAO;
    }
    
    public User getUserWithOrders(int userId) {
        User user = userDAO.getUserById(userId);
        List<Order> orders = orderDAO.getOrdersByUserId(userId);
        user.setOrders(orders);
        return user;
    }
}

Causes

• Need for separation between business logic and data access logic.
• Simplified unit testing of the data access layer.
• Easier maintenance and modification of database operations as requirements change.

Solutions

• Create a separate DAO class for each table you need to interact with.
• Use composition to gather these DAOs in a higher-level service class that handles multi-table operations.
• Implement common data access methods across these DAO classes for reusability.

Mistake: Not properly encapsulating DAO methods, leading to tight coupling with business logic.

Solution: Ensure that each DAO only exposes methods necessary for data operations, keeping them distinct from business logic.

Mistake: Overloading a single DAO class with too many responsibilities across different tables.

Solution: Create dedicated DAO classes for each table, ensuring single-responsibility for each class.