Using T4 templates to generate DataAnnotations buddy classes for EF 4 entity models

One of the more frustrating things for me lately has been leveraging the ASP.NET MVC 2 validation support using DataAnnotations with a generated Entity Framework 4 model.

Wow! That's was a mouthful!

Anyway, it's great that the MVC Framework 2.0 can leverage DataAnnotations for both validation, as well as stuff like generating label text and such. Fantastic! But, for reasons that remain retarded, Microsoft chose not to include DataAnnotations attributes with generated EF 4 entity models. You can generate the models, and the models have attributes that mirror many of the ones DataAnnotations uses, but the attributes EF 4 generates are useless for use with MVC 2.0's validation features.

DataAnnotations does have a rather interesting mechanism that you can use to add the annotations to your generated model though. You can't add the attributes directly to the generated code of course, they'd just get blown away next time the code gen ran. And you can't add the annotations to a custom partial class that extends the entities because the partial classes cannot each define two properties with the same name.

Instead, what you are supposed to do is this:

• Create a custom "meta" class (sometimes called a buddy class) that mimics the entity's public properties
    
• Annotate the public properties in the custom meta class using attributes from Data Annotations
    
• Create a partial class that extends the generated entity you are annotating
    
• Flag the custom partial entity class with an attribute called MetadataTypeAttribute to link the entity class to the meta class where the annotations live  
    

Assuming you have a generated EF 4 entity named "Ticket", this will look something like this:

[MetadataType(typeof(TicketMetadata))]
public partial class Ticket
{
    //whatever extensions I might want to the entity

    /// 
    /// Class that mimics the entity so you have a place to put data annotations
    /// 
    internal sealed class TicketMetadata
    {
        [DisplayName("Ticket Id")]
        [Required]
        public int? TicketID { get; set; }
    }
}

So, we have a way to add the annotations to our generated model, but you have to manually code this up... which is a BIG pain in the ass honestly, especially for larger entity models.

This is a job for T4, but I have no interest in modifying the core T4 templates that generate the actual EF 4 entity model. Fortunately Raj Kaimal was kind enough to blog his custom T4 template for generating metadata buddy classes.

This is a pretty slick template that auto generates meta classes by just looking at an EF 4 generated edmx file. The template is super simplistic. It doesn't account for complex types, and there isn't a built-in mechanism where you can custom override or control how the attributes are generated. For example, the T4 generates the display name by just word-splitting at the capital letters in a Pascal cased property name. So if you want a different display name, there isn't a way to override the generated display name. The template does generate the meta classes as partials, so you can implement your own extension to catch any of the complex types that the template cant automatically generate attributes for.

But because this template is so super-simple, it is a great place to start if you wanted to create your own template that can account for more complex needs specific to your own application.

In my case, I found the template useful for generating the buddy classes initially, but then I just removed the T4 template and customized the code it had generated directly. That way I could make my customizations without worrying about the T4 over-writing my changes later. The T4 saved me a boat-load of time though by just doing that initial code generation, and the code it produced was about 95% sufficient for the final product's data annotation needs.

The annoyance of using data annotations with EF models comes up so often for me that I'm seriously considering writing a Visual Studio Extension, or my own complex T4 template to deal with the problem. Sadly, I doubt I'll ever find the time to actually write the code though.