Practical ASP.NET
Using the Entity Model to Create an ASP.NET Page
What's one test of a new technology? Seeing whether you can use it with the old technology.
Over the previous two columns ("
Integrating
Entity Framework with an ASP.NET Web Site" and "
Extending
an Entity Framework Model -- and Some Limitations"), I've built a simple
Entity Framework (EF) entity model using the Northwind database that consists
of Customers, Orders and OrderDetails entities. My goal is to see how far I
can take EF and Dynamic Data to create the kind of pages that I build in the
applications for my clients.
Doing the Same Stuff
In this column, however, I want to do something simpler: Can I use EF with the
technology I already understand? For instance, the sample page that I want to
create has a drop-down list at the top that allows the user to select a company.
Implementing this with EF is both easy and a little familiar.
I start by dragging an EntityDataSource onto the page and, from its SmartTag,
select Configure Data Source. In the Configure Data Source, from the Named Connection
drop-down list, I select the name of my model. The resulting dialog lets me
select the entity that I want (Customers, in this case), apply a filter (useful
if I've used inheritance), decide which fields I want (both CustomerId and CompanyName),
and decide what updates I want to support (in this case, none). With the EntityDataSource
configured, I attach a drop-down list to the EntityDataSource just as I would
with any of the other DataSources.
But I wanted to limit the list of customers to customers in specific Regions.
I returned to my Entity Model and added an Entity mapped to a table that lists
all of the Regions in the Northwind application. I also added a Region property
to the Customer entity which I mapped to the Customer table's Region column.
I rebuilt the Web site to pick up the new entity, dropped an EntityDataSource
onto my page and configured it to use the new Region Entity. Another few clicks
put a drop-down list on the page that I tied to the Region DataSource.
This is all good because it means that a) I don't have to get everything right
the first time, and b) adding new functionality goes quickly.
To limit the customers displayed in my Customer drop-down list to the region
selected in the Region drop-down list, I go the Properties Window for the Customer
EntityDataSource. In the Properties window, I select the Where property and
click on the build button that appears (the button with the three dots) which
displays the Expression Editor dialog. In this dialog, I click the Add Parameter
button to add a parameter and configure it.
For this scenario, added a parameter to my Customer EntityDataSource and pointed
it at the Region drop-down list. Rather than attempt to write a Where clause
to use this parameter, I just checked the Autogenerate option to get the DataSource
to write its own Where clause. I did have to name the parameter myself rather
than pick a property (or column) from a drop-down list.
But I also wanted to use AJAX. So I dropped a ScriptManager and an UpdatePanel
on the page, dragged my two drop-down lists into the UpdatePanel and ran the
application. Everything worked as expected: Selecting a Region in the first
drop-down list limits the list of customers in the second drop-down list.
But, of course, other than using some new controls (and working with the Expression
Editor), I haven't really used the new technology. In fact, I actually had to
avoid some EF functionality. Had I created an association between the Customers
and Region entities, I would have to omit adding a Region property to the Customers
entity (you can't have a column in the table that's referenced both as a property
and as part of an association). Without a Region property on the Customers entity,
I wouldn't have been able to create a parameter in the EntityDataSource that
used the Customers' Region property to limit the list of Customers.
So the next step is to start actually using the technology the way it was intended.
That's for my next column.
About the Author
Peter Vogel is a system architect and principal in PH&V Information Services. PH&V provides full-stack consulting from UX design through object modeling to database design. Peter tweets about his VSM columns with the hashtag #vogelarticles. His blog posts on user experience design can be found at http://blog.learningtree.com/tag/ui/.