How to get the primary key from a table without making a second trip?

Question

How would I get the primary key ID number from a Table without making a second trip to the database in LINQ To SQL?

Right now, I submit the data to a table, and make another trip to figure out what id was assigned to the new field (in an auto increment id field). I want to do this in LINQ To SQL and not in Raw SQL (I no longer use Raw SQL).

Also, second part of my question is: I am always careful to know the ID of a user that's online because I'd rather call their information in various tables using their ID as opposed to using a GUID or a username, which are all long strings. I do this because I think that SQL Server doing a numeric compare is much (?) more efficient than doing a username (string) or even a guid (very long string) compare. My questions is, am I more concerned than I should be? Is the difference worth always keeping the userid (int32) in say, session state?

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@RedFilter provided some interesting/promising leads for the first question, because I am at this stage unable to try them, if anyone knows or can confirm these changes that he recommended in the comments section of his answer?

Answer 1

If you have a reference to the object, you can just use that reference and call the primary key after you call db.SubmitChanges(). The LINQ object will automatically update its (Identifier) primary key field to reflect the new one assigned to it via SQL Server.

Example (vb.net):

  Dim db As New NorthwindDataContext
  Dim prod As New Product
  prod.ProductName = "cheese!"
  db.Products.InsertOnSubmit(prod)
  db.SubmitChanges()
  MessageBox.Show(prod.ProductID)

You could probably include the above code in a function and return the ProductID (or equivalent primary key) and use it somewhere else.

EDIT: If you are not doing atomic updates, you could add each new product to a separate Collection and iterate through it after you call SubmitChanges. I wish LINQ provided a 'database sneak peek' like a dataset would.