Question

My entity classes are like this:

public class A{  
  [Key]  
  string Name; 
  B DefaultB;  
  ICollection<B> Bs;  
}

public class B{  
  [Key]
  int Key;
}

public class C{  
  [Key]  
  string Key; 
  A MyA;
  B MyB;  

  public C(A a,B b){  
    MyA=a;  
    MyB=b;
  } 
}  

public class MyDbContext:DbContext{  
  public DbSet<A> As
  public DbSet<B> Bs
  public DbSet<C> Cs
}  

The testing code is like this:

main(){
  A a=new A();  
  B b=new B();  
  a.addB(B);  
  MyDbContext m=new MyDbContext();  
  m.As.Add(a);  
  m.SaveChanges();
  C c=new C(a,b);
  m.Cs.Add(c);
  m.SaveChanges();
}

The problem is when I add the c to the context, it tries to add a and b again in the context, which causes an exception because they are already in it. This should not happen.

How can I solve this?

Thanks

Answer 1

If you do this all in a single context you only need one single SaveChanges at the end:

A a=new A();  
B b=new B();  
a.addB(B);

using (MyDbContext m = new MyDbContext())
{
   m.As.Add(a);  

   C c=new C(a,b);
   m.Cs.Add(c);

   m.SaveChanges();
}

EF will only create a single A and a single B and use both in the relationships in b and in c.

If you need – for some reason – to create two contexts you must reattach the entites you inserted in the first context to avoid a second INSERT and duplication in the database:

A a=new A();  
B b=new B();  
a.addB(B);

using (MyDbContext m = new MyDbContext())
{
   m.As.Add(a);  
   m.SaveChanges();
}

// ...

using (MyDbContext m2 = new MyDbContext())
{
   m2.As.Attach(a);
   m2.Bs.Attach(b);

   C c=new C(a,b);
   m2.Cs.Add(c);

   m2.SaveChanges();
}