How to add a dynamic property to my OData response which lives outside the Entity Type and is not included in the EDM Model?

[sihanook]

I have a generically typed OData controller that is able to handle requests from multiple routes/entities. This is hooked up using custom IApplicationFeatureProvider<ControllerFeature> and IControllerModelConvention implementations, similar to the way it is described in the blog (in Approach 2).

My EDM model has 2 entities: one and two. They have different properties. I have this setup to access these entity sets by using routes like:

• /api/one
• /api/two
• /api/one?$filter=id eq 2&$select=id,name&$count=true

The current API response looks some like below:

{
  "@odata.context": "http://localhost/api/$metadata#one(Id,Name)",
  "@odata.count": 1,
  "value": [
    {
      "Id": "2",
      "Name": "My name 2"
    }
  ]
}

The code in this above state is available at the location here: https://github.com/sihanook/dynamicodataroutingsample

This all works fine except that now I want to add an extra property with some additional metadata about the result set to the response. So, I want the response from the API to look like below, but still be able to use the routes as above:

{
  "@odata.context": "http://localhost/api/$metadata#one(Id,Name)",
  "@odata.count": 1,
  "value": {
    "content": [
      {
        "Id": "2",
        "Name": "My name 2"
      }
    ],
    "additionalmetadata": {
      "someProperty1": "someValue 1",
      "someProperty2": "someValue 2"
    }
  }
}

What is the correct way to achieve this in .NET Core OData? I am using version 9.0.0.

Assemblies affected

I am using "Microsoft.AspNetCore.OData" version "9.0.0".

[sihanook]

I tried the approach in this PR: sihanook/dynamicodataroutingsample#3

Note how with this approach, the response no longer looks like an OData response and "odata.count" is missing, although the rest of the response looks like what I want:

/api/one?$filter=id eq 2&$select=id,name&$count=true

{
  "content": [
    {
      "Id": 2,
      "Name": "B"
    }
  ],
  "metadata": [
    "One",
    "Two"
  ]
}

[xuzhg]

We triaged that this requirement is not compliant to the OData spec. Please follow up the OData spec if you want to build the services to server more clients.

BTW, for the expected payload, do you accept to achieve it using OData instance annotation?

[mikepizzo]

Note that the format that you propose is not OData compliant, which is why it's hard to generate/support from the OData libraries.

The OData conformant way to add additional metadata to the result set is similar to how odata-defined metadata (like context and count) is added to the payload, which is through annotations.

Representing custom metadata about the result set in an OData compliant manner would look something like:

{
  "@odata.context": "http://localhost/api/$metadata#one(Id,Name)",
  "@odata.count": 1,
  "@my.additionalmetadata": {
      "someProperty1": "someValue 1",
      "someProperty2": "someValue 2"
   },
  "value":  [
    {
      "Id": "2",
      "Name": "My name 2"
    }
  ]
}

Because this is using OData-defined annotation mechanisms for adding metadata to the response it is easier to represent and support through the OData libraries.

Is there a reason that using this annotation mechanism doesn't work for your scenario?

[sihanook]

Ah I think the annotations would also work for me. @mikepizzo what is the correct way to add custom annotations in the .NET Core OData library? It looks like I could do it with a custom serializer that overrides the CreateResource method. Is that the best way to do it?

Thanks!

[xuzhg]

Ah I think the annotations would also work for me. @mikepizzo what is the correct way to add custom annotations in the .NET Core OData library? It looks like I could do it with a custom serializer that overrides the CreateResource method. Is that the best way to do it?

Thanks!

I have a PR to support instance annotation container at C# model class. It's still in review and before it's avaiable, yes, you can customize the serializer to achieve it.

[sihanook]

@xuzhg if you have any examples of achieving this through a custom serializer, please share it. From a quick look, the ODataResourceSerializer seems to support this at a per-entity level, and not at the aggregate level, but I might be missing something. I'll dig more though.