chapter 14 Try It Out pg 505 step #4

arodrigu12 · April 20th, 2011, 08:07 PM

My question concerns the Where clause: Where="it.PhotoAlbum.Id = @photoAlbumId" (see code below)

Why is it that the Where clause uses the property PhotoAlbum from the EntitySetName Pictures to get to Id field of table PhotoAlbum i.e. it.PhotoAlbum.Id. Why doesn't it use it.PhotoAlbumId directly, from table Pictures, in setting up the Where clause? What am I missing here?

Thanks ahead of time for any help offered.

Code:

</asp:ListView>
<asp:EntityDataSource ID="EntityDataSource1" runat="server" ConnectionString="name=PlanetWroxEntities"
DefaultContainerName="PlanetWroxEntities" EnableDelete="True" EnableFlattening="False"
EnableInsert="True" EntitySetName="Pictures" Where="it.PhotoAlbum.Id = @photoAlbumId">
<WhereParameters>
<asp:QueryStringParameter Name="PhotoAlbumId" QueryStringField="PhotoAlbumId" Type="Int32" />
</WhereParameters>
</asp:EntityDataSource>
</asp:Content>

Imar · April 21st, 2011, 02:52 AM

Hi there,

It's an Object Orientation versus a relational database approach that made me prefer one over the other:

I think this feels much more natural:

Code:

it.PhotoAlbum.Id

than this:

Code:

it.PhotoAlbumId

if you look at it from an Object Orientation perspective. In an OO world, a Picture has a PhotoAlbum property that refers to the album it belongs to. The fact it needs stuff like Ids to create the relation in the database is a side issue ;-), let alone the fact it needs a foreign key property called PhotoAlbumId which is merely there for optimization.

In earlier versions of EF, a Picture wouldn't even have a PhotoAlbumId property and you had to do some weird stuff to associate them (not query them as in your example). In EF4, the foreign property was added to make assigning one object to another easier. E.g.:

Code:

myPicture.PhotoAlbumId = 3;

rather than:

Code:

myPicture.PhotoAlbum = context.Albums.Where(p => p.Id == 3).First();

The inclusion of the foreign key property is done to optimize performance and make updates easier.

But, both solutions work fine. So it's perfectly OK to use PhotoAlbumId if that feels more natural to you.

Hope this helps.

Cheers,

Imar

arodrigu12 · April 21st, 2011, 10:35 AM

Thanks, Imar. Your explanation makes perfect sense. By the way, I am getting a lot out of your book. I highly recommended it to any beginner like me.

Imar · April 21st, 2011, 02:28 PM

You're welcome.

Quote:

By the way, I am getting a lot out of your book. I highly recommended it to any beginner like me.

That's good to hear. Tell all your friends, colleagues and Amazon.com.... ;-)

Imar