Requiring an Id from SaveChanges, whilst saving? Entity Framework

I am inserting a record into the database, which looks like this:

class Transaction
{
   int Id;
}

What I want, is when I insert this object, I want to create another record, like this:

class TransactionUpdate
{
   int StartingTransactionId;
   int EndingTransactionId;
}

What I have so far, is a loop in my SaveChanges on the DbContext, which takes new Transaction objects that will be created and creates TransationUpdate objects and attaches these to the DbContext.

public override int SaveChanges()
 {
   foreach(var entry in this.ChangeTracker.Entries())
   {
     if(entry.Entity is Transaction)
     {
       var update = new TransactionUpdate();
       update.StartingTransactionId = ((Transaction)entry.Entity).PreviousTransactionId;
       update.EndingTransactionId = ((Transaction)entry.Entity).Id; // This is zero because the entity has not been inserted.
       this.TransactionUpdates.Add(update);
     }
   }
 }

The problem is, I cannot properly create a TransactionUpdate because I do not have 'EndingTransactionId', or, the Id of the Transaction I am currently inserting.

How can I solve this problem?

Many Thanks.

SOLVED

I have done what Ladislav suggested and am now creating a list of items to add, along with references to the objects that are required to insert them. Thus:

    public override int SaveChanges()
    {
        var transactionUpdatesToAdd = new List<Tuple<TransactionUpdate, Transaction>>();

        foreach (var entry in this.ChangeTracker.Entries<Transaction>())
        {
            if (entry.State == EntityState.Added)
            {
                var update = new TransactionUpdate();
                update.StartingTransactionId = ((Transaction)entry.Entity).PreviousTransactionId;
                transacti开发者_开发百科onUpdatesToAdd.Add(new Tuple<TransactionUpdate, Transaction>(update, entry.Entity));
            }
        }

        using(var scope = new TransactionScope())
        {
         // Save new Transactions
         base.SaveChanges();

         // Update TransactionUpdates with new IDs
         foreach (var updateData in transactionUpdatesToAdd)
         {
             updateData.Item1.EndingTransactionId = updateData.Item2.Id;
             this.TransactionUpdates.Add(updateData.Item1);
         }

         // Insert the new TransactionUpdate entities.
         return base.SaveChanges();
        }

Based on your description I guess you are using autogenerated Id in database. You will not receive this Id befere executing SaveChanges on the context. You have to divide operation into two separate modifications:

public override int SaveChanges()  
{  
   // call base context saving operation to insert all Transactions
   base.SaveChanges();

   foreach(var entry in this.ChangeTracker.Entries())    
   {      
      if(entry.Entity is Transaction)      
      {        
         var update = new TransactionUpdate();        
         update.StartingTransactionId = ((Transaction)entry.Entity).PreviousTransactionId;        
         update.EndingTransactionId = ((Transaction)entry.Entity).Id;      
         this.TransactionUpdates.Add(update);      
      }    
   }

   // save changes again to insert all TransactionUpdates
   base.SaveChanges();  
} 

You should wrap it into TransactionScope to perform whole saving as atomic operation.

If you haven't inserted TransactionId, you have it anyway in your object. Pass your object as parameter to an overloaded method SaveChanges and use it to pass the Id