ado.net - Entity Framework model first error on mapping table -

i'm new entity framework , when i'm creating sample projects got following error .

" entity type 'studentsubject' not mapped."

following table diagram.

the issue is not generating mapping studentsubject holds records of many many mapping of student , subject tables.

but if introduce new column id table studentsubject , make primary key , remove composite primary key works. want know why happening , way did rite way in model first approach.

i can't see code how map it. note requires no fluent api mapping

public class student {     public int id { get; set; }     public string firstname {get; set; }     public string lastname {get; set; }      public virtual icollection<studentsubject> studentsubjects { get; set; } }  public class subject {     public int id { get; set; }     public string name {get; set; }      public virtual icollection<studentsubject> studentsubjects { get; set; } }  public class studentsubject {     [key]     [column(order = 1)]     public int studentid { get; set; }      [key]     [column(order = 2)]     public int subjectid{ get; set; }      public virtual student student { get; set; }     public virtual subject subject { get; set; } }