How to use Delta from Microsoft ASP.NET Web API OData with Code First\JsonMediaTypeFormatter

Question

What is the issue?

I am trying to enable patching in my ASP.net web api app. I\'m using code first entity framework.

I have the following me

Answer 1

Interesting, it looks like Delta<T> with int members doesn't work in JSON.

Unfortunately, Delta<T> was created specifically for OData. If Delta<T> appears to be working with any formatter other than OData, it's a coincidence rather than being intentional.

The good news though is that there's nothing stopping you from defining your own PATCH format for JSON, and I'd be surprised if no one has already written one that works better with Json.NET. It's possible that we'll revisit patching in a future release of Web API and try to come up with a consistent story that works across formatters.

Answer 2

Thanks to Youssef for investigating and discovering why things weren't working. Hopefully that can get solved down the line.

I managed to crack this myself in the end after poring over the oData package source. I chose to implement another MediaTypeFormatter that wraps up the logic as it provides easy access tio HttpContent, but there are other ways to achieve this.

The key part was figuring out how to interpret the code first model, see the commented line below:

public override Task<object> ReadFromStreamAsync(Type type, Stream readStream, HttpContent content, IFormatterLogger formatterLogger)
{
    var builder = new ODataConventionModelBuilder();

    // This line will allow you to interpret all the metadata from your code first model
    builder.EntitySet<EfContext>("EfContext");

    var model = builder.GetEdmModel();
    var odataFormatters = ODataMediaTypeFormatters.Create(model);
    var delta = content.ReadAsAsync(type, odataFormatters).Result; 

    var tcs = new TaskCompletionSource<object>(); 
    tcs.SetResult(delta); 
    return tcs.Task; 
}

Hope this saves someone some trouble!