How do I map a composite primary key in Entity Framework 4 code first?

I'm getting to grips with EF4 code first, and liking it so far. But I'm having trouble mapping an entity to a table with a composite primary key.

The configuration I've tried looks like this:

public SubscriptionUserConfiguration()

    {
                Property(u => u.SubscriptionID).IsIdentity();
                Property(u => u.UserName).IsIdentity();
    }

Which throws this exception: Unable to infer a key for entity type 'SubscriptionUser'.

What am I missing?

You could also use

HasKey(u => new { u.SubscriptionID, u.UserName });

Edit:

One limitation I have found is that the following do not work:

public ProjectAssignmentConfiguration()
{
    HasKey(u => u.Employee.EmployeeId);
    HasKey(u => u.Project.ProjectId);
}

or

public ProjectAssignmentConfiguration()
{
    HasKey(u => new { u.Employee.EmployeeId, u.Project.ProjectId });
}

So how do you set up an entity where the join table has a primary key that is composed of foreign keys?

I will try to explain it step by step, using the following Entity

public class Account
{
    public int AccountId1 { get; set; }
    public int AccountId2 { get; set; }
    public string Description { get; set; }
}

1. Create a class derived from the EntityTypeConfiguaration<TEntity> Object to override the conventions

   class AccountEntityTypeConfiguration : EntityTypeConfiguration<Account>
   {
   
       public AccountEntityTypeConfiguration()
       {
         // The Key
         // The description of the HasKey Method says
         // A lambda expression representing the property to be used as the primary key.
         // If the primary key is made up of multiple properties then specify an anonymous type including the properties.
         // Example C#: k => new { k.Id1, k.Id2 }
         // Example VB: Function(k) New From { k.Id1, k.Id2 }
         this.HasKey(k => new { k.AccountId1, k.AccountId2 } );  // The Key
   
         // Maybe the key properties are not sequenced and you want to override the conventions
         this.Property(p => p.AccountId1).HasDatabaseGeneratedOption(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.None);
         this.Property(p => p.AccountId2).HasDatabaseGeneratedOption(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.None);
   
         this.Property(p => p.Description).IsRequired();  // This property will be required
         this.ToTable("Account");  // Map the entity to the table Account on the database
       }
   }
   

2. When create the class derived from the DbContext Object, override OnModelCreating Method and add a new AccountEntityTypeConfiguration object to the Configurations of the model Builder.

   public class MyModelAccount : DbContext
   {
       public DbSet<Account> Accounts { get; set;}
   
       protected override void OnModelCreating(DbModelBuilder modelBuilder)
       {
           // Add a new AccountEntityTypeConfiguration object to the configuration of the model, that will be applied once the model is created. 
           modelBuilder.Configurations.Add(new AccountEntityTypeConfiguration());
       }
   
   }
   

Hope it helps you!