Thursday, December 27, 2007

Linq To Sql, Attach, Using a DateTime as Timestamp and large bottle of ibuprofren!!!

Scenario:

You use LINQ to SQL (L2S) as your ORM, you create a business layer that supplies a list of customer entities to the presentation layer. The UI presents the data in a gridview where the user can update the info.

When the user clicks update, it passes a new instance of the entity to your BL method. Now you might be tempted to think that you simply use the Attach method of the Customers table and everythings fine and dandy. Oh that it were true. Be ready for:

"System.InvalidOperationException: An entity can only be attached as modified without original state if it declares a version member or does not have an update check policy."

The entity being passed to your BL here is *not* the same one that went out. It is a new one created by the GridView as part of the update. Thus, it has no idea of the original context and your new context has no idea of it's original state.

If you have add a timestamp column that can be used by L2S to determine if the row has changed, you can make things work fine. This has some advantages in that it reduces the amount of T-Sql generated to perform the update.

// Business Layer code to get the list and perform the update.
[System.ComponentModel.DataObjectMethod(System.ComponentModel.DataObjectMethodType.Select)] 
public static List ListCustomers()
{
    using (SandboxDataContext dataContext = new SandboxDataContext())
    {
        var custs = from c in dataContext.Customers orderby c.CustomerName select c ;
        return custs.ToList();
    }
}

[System.ComponentModel.DataObjectMethod(System.ComponentModel.DataObjectMethodType.Update)]
public static void UpdateCustomer(Customer c)
{
    using (SandboxDataContext dataContext = new SandboxDataContext())
    {
        dataContext.Customers.Attach(c, true);
        dataContext.SubmitChanges();
    }
}

Note: You must use the Attach(entity, true) overload to actually save the changes.
Note: Make sure you configure the DataKeys property of your grid to include the primary key and the timestamp. Otherwise, you get a ChangeConflictException -- Row not found or changed.

-- T-Sql generated by L2S

exec sp_executesql N'UPDATE [dbo].[Customers]
SET [CustomerName] = @p2
WHERE ([CustomerKey] = @p0) AND ([TimeStamp] = @p1)

SELECT [t1].[TimeStamp]
FROM [dbo].[Customers] AS [t1]
WHERE ((@@ROWCOUNT) > 0) AND ([t1].[CustomerKey] = @p3)',

N'@p0 int,
@p1 timestamp,
@p2 varchar(40),
@p3 int',
@p0=18,
@p1='0x00000000000007D1',
@p2='Acme Widgets, LLC',
@p3=18

Notice how only the CustomerName column is updated instead of every column. This is one of the largest advantages over stored procs. Ordinarily, you'd probably write one stored proc to update the table and it would update every column. You definitely wouldn't want to write an SP for *every* column or permutation of columns.
Instead, what's produced is a nice tight query that only changes those columns that need to be changed. IMHO, this outweighs any perceived performance issues versus stored procs.

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Wednesday, December 26, 2007

Fixing Association Lines in the LINQ to SQL Designer

There is a simple fix for when the Linq to Sql designer goes crazy with the number of points on an association line. Simply close the designer, click "Show All Files" in your solution. You should now be able to expand the underlying files used to store info for the designer. Open the dataContextName.dbml.layout file with the XML Editor. Associations are stored in an associationConnector element. Find the correct association and change the following attributes to appear as follows: manuallyRouted="false" fixedFrom="NotFixed" fixedTo="NotFixed" If manuallyRouted doesn't appear, don't worry about it. Save everything and close the file. Reopen the designer and your line still looks weird but as soon as you move one of the objects around, it should fix itself.

Friday, December 21, 2007

Steelers Game!!

Went to STL for the Rams-Steelers game. As we we're driving to the hotel, you'd have thought it was a home game. There were more Steeler nation than Rams fans by far. If you ever want to see a site, Casey Hampton was stretching in front of us. At one point he was doing the exercise where you bring your knee across your body and try to touch it to the ground. You must see it for yourself! Alan Fanaca was also stretching and didn't seem to want to talk. Wish him the best next year. He's given a lot to the team over the years and deserves to get a great contract. Forget the hometown discount!!! The game was excellent! Didn't see Willie get hurt although I remember the play. The opening pass from Ben to Holmes was amazing. Need to work on our coverage. Is it just me, or does it seem that you can throw under DeShea pretty easily?!