How to update related entities in Entity Framework

Question

I have an MVC project and using Entity Framework Code First and POCO objects for the database. For example:

public class ClassA
{
  public int? Id {get; set;         


        

Answer 1

I solved it by changing ClassA with a foreign key to ClassB, and setting the value of BId:

public class ClassA
{
  public int? Id {get; set;}
  public string Name { get; set;}
  public int BId {get;set;} // foreign key to B
  public virtual ClassB B {get; set;}
}

Why does this foreign key property does the job instead of the generated foreign ky of EF?