Question

This question is in two parts:

A) Is my design wrong? If so, what would be the proper way to do this?

B) If my design is right, how do I implement it?

I have a data class that consists of multiple instances of a component class. An abstraction of the class structure would be this:

In this example I have only two components in the container but in reality I have more. It’s also important to point out that Container cares about which Component is which – they can’t just be a list, they have individual names and meaning to the Container.

I’ve implemented the following structure in MS SQL Server 2008 to store this data:

CREATE TABLE Containers (
  ID int IDENTITY(1,1) NOT NULL,
  ComponentA int NULL,
  ComponentB int NULL,
 CONSTRAINT PK_Containers PRIMARY KEY (ID));


CREATE TABLE Components (
  ID int IDENTITY(1,1) NOT NULL,
  Foo nchar(10) NOT NULL,
  Bar nchar(10) NOT NULL,
 CONSTRAINT PK_Components PRIMARY KEY (ID));


ALTER TABLE Containers
  ADD CONSTRAINT FK_ComponentA
  FOREIGN KEY (ComponentA)
  REFERENCES Components(ID);

ALTER TABLE Containers
  ADD CONSTRAINT FK_ComponentB
  FOREIGN KEY (ComponentB)
  REFERENCES Components(ID);   

This already has a few drawbacks:

• In my model, components are unique, and can only belong to one property on one Container. In the DB, I have to enforce this manually.

• When a Container is deleted, the Components it references should also be deleted. This I have to do manually in the DB (with a trigger).

The big problem, however, is this:

I use LINQ-TO-SQL for my database access in the C# WPF MVVM desktop application I am building in Visual Studio 2010. When I pull the database tables above into the LINQ-TO-SQL designer, this is what I get:

I can manually set the relationships to One-to-One in the LING-TO-SQL designer, to stop the Component instances from having sets of type Container.
Correctly, each Container has two Component references. However, the Component instances still have two Container references, where my model requires them to have only one.

How can I implement my class design in SQL Server or how can I convince LINQ-to-SQL to interpret my database the way I intended?

Answer 1

A container has many components, but needs to reference them by name. You need a relationship table from container to component (in pseudo-sql):

create table ContainerComponents (
  ContainerID int FK references Container(ID),
  ComponentID int FK references Component(ID),
  Name string,
  -- optional - to give the relationship a meaning
  Type int FK references RelationshipType(ID)
)

To answer this question:

In my model, components are unique, and can only belong to one property on one Container. In the DB, I have to enforce this manually.

Just create a unique constraint in ContainerComponents for ContainerID and ComponentID.

And this one:

When a Container is deleted, the Components it references should also be deleted. This I have to do manually in the DB (with a trigger).

Use cascade deletes from Container to ContainerComponents to Component.