My working code can be found on Github, and a brief overview is described here.
Sorry to use a contrived example, but I could hardly use a production example, and din't have the time to think up anything else, so pizzas it is. At least it's not coffee.
Here is a typical implementation of the pattern:
public interface IPizza
{
Guid? Id { get; set; }
int Size { get; set; }
Quantity Cheese { get; set; }
Quantity Tomato { get; set; }
decimal Cost { get; }
Order Order { get; set; }
}
public class Pizza : Entity, IPizza
{
public virtual int Size { get; set; }
public virtual Quantity Cheese { get; set; }
public virtual Quantity Tomato { get; set; }
public virtual Order Order { get; set; }
public static IPizza Create(int size, Quantity cheese, Quantity tomato)
{
// Create code...
}
public virtual decimal Cost
{
// Calculate cost...
}
}
public class ToppingDecorator : Entity, IPizza
{
public virtual IPizza BasePizza { get; set; }
public virtual Order Order { get; set; }
public ToppingDecorator(IPizza basePizza)
{
Id = Guid.NewGuid();
BasePizza = basePizza;
}
public virtual int Size
{
get { return BasePizza.Size; }
set { BasePizza.Size = value; }
}
public virtual Quantity Cheese
{
get { return BasePizza.Cheese; }
set { BasePizza.Cheese = value; }
}
public virtual Quantity Tomato
{
get { return BasePizza.Tomato; }
set { BasePizza.Tomato = value; }
}
public virtual decimal Cost
{
get { return BasePizza.Cost; }
}
}
public class PepperoniDecorator : ToppingDecorator
{
public virtual bool ExtraSpicy { get; set; }
public PepperoniDecorator(IPizza basePizza, bool extraSpicy)
: base(basePizza)
{
ExtraSpicy = extraSpicy;
}
public override decimal Cost
{
get
{
// Add to cost...
}
}
}
public class OliveDecorator : ToppingDecorator
{
public virtual OliveColour Colour { get; set; }
public OliveDecorator(IPizza basePizza, OliveColour colour) : base(basePizza)
{
Colour = colour;
}
public override decimal Cost
{
get
{
// Add to cost...
}
}
}
public class Order : Entity
{
public virtual string CustomerName { get; set; }
public virtual string DeliveryAddress { get; set; }
public virtual IList Items { get; set; }
//Create/Add methods etc...
}
When it came to the database, it was always pretty clear that there would be a Pizza table which would contain all the properties specified in the interface, and then there would be tables for each decorator which contained the particular fields they added, and also a foreign key to either a Pizza or another decorator:
USE [Master] IF EXISTS (SELECT * FROM sys.databases WHERE NAME = 'Decorator') BEGIN EXEC msdb.dbo.sp_delete_database_backuphistory database_name = N'Decorator'; ALTER DATABASE [Decorator] SET SINGLE_USER WITH ROLLBACK IMMEDIATE; DROP DATABASE [Decorator]; END GO CREATE DATABASE [Decorator] GO USE [Decorator] GO IF NOT EXISTS(SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'Pizza') BEGIN CREATE TABLE [dbo].[Pizza]( [Id] uniqueidentifier NOT NULL PRIMARY KEY, [Size] int NULL, [Cheese] int NULL, [Tomato] int NULL, [OrderId] uniqueidentifier NULL ); END GO IF NOT EXISTS(SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'PepperoniDecorator') BEGIN CREATE TABLE [dbo].[PepperoniDecorator]( [Id] uniqueidentifier NOT NULL PRIMARY KEY, [BasePizzaId] uniqueidentifier NULL, [ExtraSpicy] bit NULL, [OrderId] uniqueidentifier NULL ); END GO IF NOT EXISTS(SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'OliveDecorator') BEGIN CREATE TABLE [dbo].[OliveDecorator]( [Id] uniqueidentifier NOT NULL PRIMARY KEY, [BasePizzaId] uniqueidentifier NULL, [Colour] int NULL, [OrderId] uniqueidentifier NULL ); END IF NOT EXISTS(SELECT * FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'Order') BEGIN CREATE TABLE [dbo].[Order]( [Id] uniqueidentifier NOT NULL PRIMARY KEY, [CustomerName] nvarchar(100) NULL, [DeliveryAddress] nvarchar(200) NULL ); END GOThe trick bit was mapping between them. After several failed attempts at using table per class heirachy and table per subclass I came to the conclusion that it wasn't the way to go.
I experimented with table per concrete class using implicit polymorphism but found the limitations of that to be a major issue. Eventually the solution was found using table per concrete class using union-subclass.
Here is how the mappings look:
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
namespace="Decorator.Domain.Entities"
assembly="Decorator.Domain">
<class name="IPizza" abstract="true">
<id name="Id" column="Id" type="guid">
<generator class="assigned"/>
</id>
<many-to-one name="Order" class="Order" column="`OrderId`" cascade="save-update" />
<union-subclass name="Pizza" table ="`Pizza`" >
<property name="Size" column="`Size`" />
<property name="Cheese" />
<property name="Tomato" />
</union-subclass>
<union-subclass name="PepperoniDecorator" table ="`PepperoniDecorator`" >
<many-to-one name="BasePizza" class="IPizza" column="`BasePizzaId`" cascade="all" />
<property name="ExtraSpicy" column="`ExtraSpicy`" />
</union-subclass>
<union-subclass name="OliveDecorator" table ="`OliveDecorator`" >
<many-to-one name="BasePizza" class="IPizza" column="`BasePizzaId`" cascade="all" />
<property name="Colour" column="`Colour`" />
</union-subclass>
</class>
</hibernate-mapping>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
namespace="Decorator.Domain.Entities"
assembly="Decorator.Domain">
<class name="Order" table="`Order`">
<id name="Id" column="Id" type="guid">
<generator class="assigned"/>
</id>
<property name="CustomerName" />
<property name="DeliveryAddress" />
<bag name="Items" inverse="true" cascade="save-update">
<key column="`OrderId`"></key>
<one-to-many class="IPizza" />
</bag>
</class>
</hibernate-mapping>
I have included the Order entity for a good reason here: If you create a Pizza, decorate it with pepperoni, then decorate it with olives and save it, when you get all pizzas, it will actually return 3 pizzas! NHibernate has no way of knowing which pizza is the top level one. This could be avoided by having an IsTopLevel flag, but as pizzas will always be created in the context of an order, it makes sense to only have the orderId on the top level. A similar solution will apply to most scenarios.
