Code First: Independent associations vs. Foreign key associations?

If you want to take full advantage of ORM you will definitely use Entity reference:

public class Order
{
    public int ID { get; set; }
    public Customer Customer { get; set; } // <-- Customer object
    ...
}

Once you generate an entity model from a database with FKs it will always generate entity references. If you don't want to use them you must manually modify the EDMX file and add properties representing FKs. At least this was the case in Entity Framework v1 where only Independent associations were allowed.

Entity framework v4 offers a new type of association called Foreign key association. The most obvious difference between the independent and the foreign key association is in Order class:

public class Order
{
    public int ID { get; set; }
    public int CustomerId { get; set; }  // <-- Customer ID
    public Customer Customer { get; set; } // <-- Customer object
    ...
}

As you can see you have both FK property and entity reference. There are more differences between two types of associations:

Independent association

• It is represented as separate object in ObjectStateManager. It has its own EntityState!
• When building association you always need entitites from both ends of association
• This association is mapped in the same way as entity.

Foreign key association

• It is not represented as separate object in ObjectStateManager. Due to that you must follow some special rules.
• When building association you don't need both ends of association. It is enough to have child entity and PK of parent entity but PK value must be unique. So when using foreign keys association you must also assign temporary unique IDs to newly generated entities used in relations.
• This association is not mapped but instead it defines referential constraints.

If you want to use foreign key association you must tick Include foreign key columns in the model in Entity Data Model Wizard.

Edit:

I found that the difference between these two types of associations is not very well known so I wrote a short article covering this with more details and my own opinion about this.

Use both. And make your entity references virtual to allow for lazy loading. Like this:

public class Order
{
  public int ID { get; set; }
  public int CustomerID { get; set; }
  public virtual Customer Customer { get; set; } // <-- Customer object
  ...
}

This saves on unnecessary DB lookups, allows lazy loading, and allows you to easily see/set the ID if you know what you want it to be. Note that having both does not change your table structure in any way.