c# - EF 4.1 Code First: Many to Many -

i have 2 entities, let's , b. relation between them many-to-many have entity, let's c.

columns table a:
-id (pk) -> type int generated database
-propertya1
-propertya2
-propertya3

columns table b:
-id (pk) -> type int generated database
-description

columns table c (for table not sure if better add column id generated database previous tables):
-ida (pk , foreign key entity a)
-idb (pk , foreign key entity b)

table b has fixed values inserted on seed method (overrided). entries below:
id description
1 "some description 1"
2 "some description 2"
3 "some description 3"

from form, user introduces information related table (propertya1,...,propeprtya3) , click on button save data database.

once user clicks on button's form save data database, first following:

   a = new a(){ propertya1=something_1,                   propertya2=something_2,                   propertya3=something_3 };     context.a.add(a);    context.savechanges(); 

then after saving changes database have id generated database (i have not id before saving database), is, a.id, can proceed add entry table c doing:

  b b = this.obtainappropriateb();   c c = new c(){ ida = a.id,                  idb = b.id };    context.c.add(c);   context.savechanges(); 

my problem is:
1) cannot know a.id previous context.savechanges after context.a.add(a) because generated database.
2) if context.savechanges fails after context.c.add(c), how can rollback previous work done?:
context.a.add(a);
context.savechanges();

i cannot following because have not a.id previous savechanges:

   a = new a(){ propertya1=something_1,                   propertya2=something_2,                   propertya3=something_3 };     context.a.add(a);    b b = this.obtainappropriateb();    c c = new c(){ ida = a.id,                   idb = b.id };     context.c.add(c);     context.savechanges(); <--- call once persist changes database 

how solve this?

this not way work many-to-many relationship entity framework.

first, should not have c entity in model instead collection properties on entity a , b:

public class {     public int aid { get; set; }     public int propertya1 { get; set; }     public string propertya2 { get; set; }     public datetime propertya3 { get; set; }      public icollection<b> bs { get; set; } }  public class b {     public int bid { get; set; }      // ...      public icollection<a> { get; set; } } 

from collections ef able figure out relationship many-to-many , link table (c) needed. can define mapping details fluent api.

second, once have navigation collections don't need care ids. build object graph necessary relationships , save database:

a = new a() { propertya1 = something_1,                 propertya2 = something_2,                 propertya3 = something_3 }; a.bs = new list<b>();  b b = this.obtainappropriateb(); // must use same context instance  a.bs.add(b);  context.a.add(a); context.savechanges(); 

it create new a , insert relationship row between a , b link table.