ASP.NET 4 : Getting More Advanced with the Entity Framework (part 2) - Updates, Inserts, and Deletes

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3. Updates, Inserts, and Deletes

The Entity Framework isn't limited to querying data. You can also perform inserts, updates, and deletes. The best part is that you accomplish all this by manipulating the familiar entity objects. Once again, the Entity Framework translates your VB code into the appropriate SQL database commands. All you need to do is call the SaveChanges() method of the context object. In the current example, that means a quick call to NorthwindEntities.SaveChanges() will commit any recent changes to the database.

Figure 2 shows a web page that puts this into practice. It includes a grid of products and three buttons: one that changes the currently selected record, one that inserts a new record, and one that deletes the selected record.

Figure 2. Modifying a database with the Entity Framework

To update a record, you need to first retrieve the right entity object. The technique you use depends on the exact task you're performing, but you might get your object by iterating through a collection, following a navigation property, or performing a LINQ expression, as shown here.

However, there's one minor trick. All LINQ expressions return an iterator that allows you to traverse a collection of objects. If you have an expression that you know matches just one record (for example, one that uses the Where keyword on a unique key value, such as ProductID), you can use the Single() method. This executes the query and returns the one and only matching object.

The following code selects a specific product (using a LINQ expression), updates the stock numbers, and saves the change. The two code statements in bold trigger immediate database operations and so require exception handling:

Protected Sub cmdUpdate_Click(ByVal sender As Object, ByVal e As EventArgs) _
Handles cmdUpdate.Click

' Only allow updating if a record is currently selected.
If gridProducts.SelectedIndex <> −1 Then
' Use a LINQ expression to find the selected product.

Dim selectedProductID As Integer = Val(gridProducts.SelectedDataKey.Value)
Dim matches = From p In entities.Products
Where p.ProductID = selectedProductID
Select p

' Execute the query and return the entity object.
Dim product As Product = matches.Single()

' Change the entity object.
product.UnitsInStock -= 1

' Commit the changes back to the database.
End If
End Sub


SaveChanges() also returns an integer that indicates how many records were updated. So if you get a result of 0, you know that no operations were performed.

Inserting a new record is nearly as easy. The best approach is to use the shared factory method of the appropriate entity class. So if you want to create a new product, call Product.CreateProduct(). Then, add the new object to the NorthwindEntities.Products collection using the AddObject() method, and commit the update with SaveChanges():

Protected Sub cmdInsert_Click(ByVal sender As Object, ByVal e As EventArgs) _
Handles cmdUpdate.Click

' The CreateProduct() method requires the three non-nullable Product fields
' as arguments: ProductID, ProductName, and Discontinued. However, the
' ProductName isn't actually used--when the update is finished, it's replaced
' by the automatically generated ID that the database creates.
Dim newProduct As Product
newProduct = Product.CreateProduct(0, "Thick-As-Cement Milkshake", False)

' You can now set additional properties that aren't required.
newProduct.CategoryID = 1
newProduct.UnitsInStock = 10
newProduct.UnitPrice = 15.99D

' Finally, commit the changes and insert the record in the database.
End Sub

Finally, a delete operation is the simplest data operation of all. You simply need to call the DeleteObject() method of the appropriate collection, followed by SaveChanges(). The following code retrieves the currently selected product object and removes it from the database using this approach:

Protected Sub cmdDelete_Click(ByVal sender As Object, ByVal e As EventArgs) _
Handles cmdDelete.Click

' Check if a row is selected.
If gridProducts.SelectedIndex <> −1 Then
' Use a LINQ expression to find the selected product.
Dim selectedProductID As Integer = Val(gridProducts.SelectedDataKey.Value)
Dim matches = From p In entities.Products
Where p.ProductID = selectedProductID
Select p

' Execute the query and return the entity object.
Dim product As Product = matches.Single()

' Delete the record from the database.

' Clear the selection (which may now be pointing to a different row.)
gridProducts.SelectedIndex = -1
End If
End Sub

The Entity Framework uses a fairly sophisticated change tracking system to make these operations work.

When you call SaveChanges(), the context object attempts to commit all of the changes in the data that it's tracking, including updates, deletions, and insertions. If you have related records, the Entity Framework will apply the changes in an order that makes sense (for example, it won't attempt to insert a child record before inserting a linked parent). This ability is more interesting if you use the Entity Framework in a rich desktop application, where you might keep the entity objects in memory for a long period of time, and periodically commit a batch of updates. But in a web application, where your page lives for only a few seconds (or less), you'll almost always create a new context object, perform all your database operations immediately, and then discard it.


When you call SaveChanges(), all the changes are performed as part of a single transaction. If an error occurs, the entire process is rolled back, but your objects remain in their current state, with any new and edited information. That means you can programmatically resolve the problem and call SaveChanges() again.

4. Managing Concurrency

Update errors can be caused by all the usual factors—a timeout waiting for a connection, a network error, and so on—but the most common update error is a concurrency problem.

There are two overall strategies you can use to deal with concurrency issues: you can try to avoid concurrency problems before they happen, or you can try to resolve concurrency issues after they occur.

By default, the Entity Framework uses the "last-in-wins" updating strategy. This means that new updates always succeed, but they may wipe out another user's changes. However, this doesn't pose the same problem that it does with the SqlDataSource control . That's because the Entity Framework doesn't apply old values. If you change a single field and call SaveChanges(), the Entity Framework uses an update command that modifies just that field.

The Entity Framework's updating strategy can still cause a problem if two users are updating the same record at once and they are editing the same fields (or, if the changes they're making to different fields aren't consistent). Depending on the way you're using the database and the frequency of edits, this may not be a significant concern for your application. (After all, the time taken to make and commit an edit in an ASP.NET application is very small, unlike a desktop application.) But if you need to catch potential problems that involve overlapping edits, you can configure the Entity Framework to use stricter concurrency checking.

Here's how it works. Using the data model designer, you choose the fields that must match in order for an update to succeed. For example, you might decide that a Customer record update should be allowed to continue only if the ContactName and CompanyName fields haven't been changed by another user since the record was retrieved. To do this, start by clicking the ContactName field in the Customer table box. Then, change the Concurrency Mode property in the Properties window from None to Fixed. Finally, repeat this process for the CompanyName field.

Now, if you attempt to commit a change and it overlaps with records that another user has changed, the edit will fail with an OptimisticConcurrencyException. At this point, you can try to resolve the problem or give the user the choice of reapplying the update.
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