Domain Objects Persistence Pattern for .NET

Abstract

When it comes to developing object oriented applications that use relational databases, the domain object design has to be consistent with the database design. This make them easier to understand because they represent real-life "entities" and their relationships with each other. Therefore, in many situations, the domain objects are "mapped" to the relational database tables and relationships between tables. However, it is very easy to get this mapping wrong and end up with an undesirable domain object design. A good design for domain objects requires a solid understanding of object oriented and relational fundamentals on the part of developers.

Domain Objects Persistence Pattern attempts to provide a solution for domain object mapping to the relational databases that decouples the domain objects from the persistence logic. The domain objects in this pattern are unaware of the objects that persist them because the dependency is only one-way (from persistence objects to domain objects). This makes the domain objects design much simpler and easier to understand. It also hides the persistence objects from other subsystems in the application that are using the domain objects. This also works in distributed systems where only the domain objects are passed around. In this context, an attempt is made to incorporate the Factory Pattern into this pattern to help decouple domain objects and persistence logic.

Scope

Problem Definition

If domain objects are not separated from the rest of the application, we end up with duplication of code everywhere. Similarly, if domain objects are not separated from the persistence code, we face situations where any subsystem using the domain objects also knows and depends on the persistence objects. And, any change in persistence objects affects the entire application, hence a bad design.

Solution

In separating the persistence logic from domain objects, we ensure that the domain objects have no dependency on the persistence code. This allows the domain objects to become available in environments where we don't even want to expose our persistence code.

Sample Code

In this sample, we will look at a Customer object from Northwind database mapped to the "Customers" table in the database.

public class Customer {
    // Private data members
    String      _customerId;
    String      _companyName;
    String      _contactName;
    String      _contactTitle;
    public Customer() {}
    // Properties for Customer object
    public String CustomerId {
        get { return _customerId; } set { _customerId = value;}
    }
    public String CompanyName {
        get { return _companyName; } set { _companyName = value;}
    }
    public String ContactName {
        get { return _contactName; } set { _contactName = value;}
    }
    public String ContactTitle {
        get { return _contactTitle; } set { _contactTitle = value;}
    }
}
public interface ICustomerFactory 
{
    // Standard transactional methods for single-row operations
    void Load(Customer cust);
    void Insert(Customer cust);
    void Update(Customer cust);
    void Delete(Customer cust);
    // Query method to return a collection
    ArrayList FindCustomersByState(String state);
}
public class CustomerFactory : ICustomerFactory
{
    // Standard transactional methods for single-row operations
    void Load(Customer cust) { /* Implement here */ }
    void Insert(Customer cust) { /* Implement here */ }
    void Update(Customer cust) { /* Implement here */ }
    void Delete(Customer cust) { /* Implement here */ }
    // Query method to return a collection
    ArrayList FindCustomersByState(String state) { /* Implement here */ }
}

Below is an example of how a client application will use this code.

    public class NorthwindApp
    {
        static void Main (string[] args) {
            Customer cust = new Customer();
            CustomerFactory custFactory = new CustomerFactory();
            // Let's load a customer from Northwind database.
            cust.CustomerId = "ALFKI";
            custFactory.load(cust);
            // Pass on the Customer object
            FooBar(cust);
            // custList is a collection of Customer objects             
            ArrayList custList = custFactory.FindCustomersByState("CA");
         }
    }

Conclusion

Using the Domain Objects Persistence pattern, we have extracted the persistence code out of the Customer object. This has made Customer object more object-oriented and simpler to understand because its object model is closer to the data model in the database. And, finally, we have enabled the Customer object to be passed around to different parts of the application (or even to distributed applications through .NET Remoting) without exposing its persistence code.