LINQ - How to read ID which is Autonumber after submitting the data - linq

Question

I am working on LINQ. and in one transaction I have to update three different tables.
e.g A(A1,A2,A3) B(B1,B2) AB(A1,B1)
here B1 is autonumber in my database. I want to submit all the changes together. so in one data context I wrote
using (BBBDataContext DC= new BBBDataContext())
{
A tba = new A()
{
A1 = this.A1,
A2 = this.A2,
A3 = this.A3,
};
DC.A.InsertOnSubmit(tba);
B tbb= new B()
{
B2 = this.B2,
};
DC.B.InsertOnSubmit(tbb);
// NOW i WONT B1(WHICH IS AUTONUMBER) FROM B SO THAT I USE IT IN MY NEXT TABLE.
AB tbab = new AB()
{
A1 = this.A1,
B1 = ??????,
};
DC.AB.InsertOnSubmit(tbab);
//FINALLY I WILL WRITE SUBMIT CHANGES SO MY ALL TABLES GET CHANGES SAME TIME
DC.SubmitChanges();
}
Que: what should I write # the place of ?????. for B1 in AB table??

Answer 1

Short answer: B
Bit longer answer:
When you create the object and add it you won't get the ID before you commit. So what you do is reference the object and not the ID property of the object.
class A {
public int Id { get;set;}
public ICollection <B> Bees { get;set;}
}
class B {
public int Id { get;set;}
public int InstanceOfAId { get;set;}
public A InstanceOfA { get;set;}
}
var a = new A();
var b = new B();
b.InstanceOfA = a;
Depending on your model you will define a relationship. In code first you could do it this way:
EntityTypeConfiguration<B> cfgB = new EntityTypeConfiguration<B>();
cfgB.HasRequired(p => p.InstanceOfA).WithMany(p => p.Bees)
.HasForeignKey(p => p.InstanceOfAId);
In EDMX you can do the same in the designer.
You will use InstanceOfA to assign to when inserting, after that InstanceOfAId will always hold the Id value. When retrieving data InstanceOfA will only be filled when requested.
You can use it like this:
var x = DC.B.Include(p=>p.A);
or
var x = DC.B.Select(p=> new { Id = p.Id, A_Id = p.A.Id});
or
var x = DC.A.Select(p=> new { Id = p.Id, BeeCount = p.Bees.Count()});