Creating a custom property in Entity Framework

Question

I have a database I'd like to create an entity from, and then generate RESTful output.

My objective is to add a property to one of the tables once it becomes an entity. The data for that property would be one I'd come up with through calculations done on a few different fields in the table. From there, the code generator would create RESTful output like it normally does.

I have managed to be able to update the SSDL, CSDL, and the mapping sections of the edmx file along with using the SampleEdmxCodeGenerator as a custom tool. When I have all the sections in the edmx file filled out with my custom property, the svc fails because (I'm assuming) the property doesn't exist in the database. If I leave the property out of the SSDL, but put it in the client schema (CSDL) and the mapping section, I can't build my project.

I've modified the partial class and added to it, but the problem there is that I need to populate the methods on the creation time of the class, and I haven't been able to do that yet.

Am I headed in the right direction, or is this not possible? It seems like I should be able to do this with minimal effort, but I keep hitting walls.

Answer 1

I think you're taking detours to get where you want. I haven't used either of these approaches (recently), so they might not do exactly what you're after, but you could try this:

1. Create a partial class file right next to the .edmx model, which has the same name as your entity.
2. In it, specify the property you want as a read-only property, that does the calculations on each get.

Answer 2

Partial Classes and Partial methods were the first part of my answer. What I'm essentially trying to do I can't do. I can manipulate data that is returned by using partial methods and partial classes. I can plug the On*methodname*Changed() method to format the data how I'd like it to be shown, but that only gets me part way to my desired result.

What I would also like to do, is create a property c, which doesn't exist as a column in the database (and therefore does not exist in my entity), calculated from a couple different properties in the database (say a and b), and then add property c to the entity framework class. In doing this, I figured it would then get generated into the RESTful webservice output.

A problem that occurs comes from the need for the class to update any changes you make, and have it propagate back to the data source. I didn't care about that, because I want my property to be read only. From what I've gathered this isn't possible.

For reference, these two posts really helped: Adding custom property to Entity Framework class (I can only post one url currently, so here is the address to the other article)

social.msdn.microsoft.com/Forums/en-US/adodotnetdataservices/thread/b7a9e01d-c5c2-4478-8f01-00f7f6e0f75f

What I've decided to do, is to expose my entity as I've done so far, then consume the RESTful service that manipulates data and reformats it, and introduces needed properties. I'll turn the results into my own data object, and use that as a datasource to be exposed by yet another RESTful web service. I think this website gives a good example on how to expose a custom datasource.

mstecharchitect.blogspot.com/2008/12/surfacing-custom-data-source-in-adonet.html

If for some reason that is too slow, I suppose I could just make another table in my database that has a reworking of the data, and the calculated output in a format I'm looking for. The thing I want to avoid is having my resulting client having to do any of the data manipulation since it will be on some micro devices like palms, iphones, and blackberries.

Hope that helps anyone else with the same problem. It seems that is a shortfall in the current version of Data Services, but to some extent, I'm sure they'll be addressing it in later versions. Maybe T4 and .net 4.0 will be addressing it. I'm not sure.