[DataContract]
public class Invoice : EntityBase, IIdentifiableEntity
{
    [DataMember]
    public int InvoiceId { get; set; }
    [DataMember]
    public int OrderId { get; set; }

    public Order Order { get; set; }
    public ICollection<Payment> Payments { get; set; }
    public int EntityId
    {
        get { return InvoiceId; }
        set { InvoiceId = value; }
    }
}

[DataContract]
public class Payment : EntityBase, IIdentifiableEntity
{
    [DataMember]
    public int PaymentId { get; set; }

    public ICollection<Invoice> Invoices { get; set; }
    public int EntityId
    {
        get { return PaymentId; }
        set { PaymentId = value; }
    }
}

[DataContract]
public class InvoicePayment
{
    [DataMember]
    public int PaymentId { get; set; }

    [DataMember]
    public int InvoiceId { get; set; }

    public Payment Payment { get; set; }
    public Invoice Invoice { get; set; }
}

//In the model builder
modelBuilder.Entity<InvoicePayment>().HasKey(k => new
{
    k.InvoiceId,
    k.PaymentId
});
modelBuilder.Entity<InvoicePayment>()
    .HasRequired(p => p.Payment)
    .WithMany()
    .HasForeignKey(p => p.PaymentId);

modelBuilder.Entity<InvoicePayment>()
    .HasRequired(p => p.Invoice)
    .WithMany()
    .HasForeignKey(p => p.InvoiceId);