[NancyFx] Inheritance

Description

Currently the Nancy Generator does not support inheritance at all. I'll briefly discuss the current status of nancy w.r.t. inheritance based on the simple Pet/Cat model found further down.

Models:

The models inherit correctly from each other:

public class Pet:  IEquatable<Pet>{...}
public sealed class Cat: Pet,  IEquatable<Cat>{...}

However, the builder of the Cat currently doesn't allow setting the Pet properties (with immutable=true):

public sealed class CatBuilder {
  public CatBuilder HuntingSkil(string value) {...}
  public Cat Build() {...} 
  ... private members....
}

Deserialization:

The /pet endpoint deserializes subtypes (such as a Cat) currently wrong, i.e. it just serializes them as a Pet and ignores all additional properties of Cat:

Post["/pet"] = parameters =>
{
    var body = this.Bind<Pet>();
    Preconditions.IsNotNull(body, "Required parameter: 'body' is missing at 'PetPost'");
    
    return service.PetPost(Context, body);
};

Serialization:

Serialization works out of the box. That is a Cat is serialized as a Cat. The only problem is, that the discriminator is not automatically set (and because of the problem with model described above, it can in fact not be set at all).

Swagger-codegen version

2.3-Snapshot

Swagger declaration file content or url

Here is the swagger file that I have used above to describe the current status:

swagger: "2.0"
info:
  version: "1.0.0"
  title: Demo


paths:
  /pet:
    post:
      summary: "post a bar"
      parameters:
        - name: body
          in: body
          required: true
          schema: 
            $ref: '#/definitions/Pet'
      responses:
        201:
          description: successful.
          schema:
            $ref: '#/definitions/Pet'
  
          
definitions:
  Pet:
    discriminator: type
    type: object
    required:
      - type
    properties:
      type:
        type: string
      name:
        type: string
        
  Cat:
    allOf:
      - $ref: '#/definitions/Pet'
      - properties: 
          huntingSkil:
            type: string

Command line used for generation

-l nancyfx --additional-properties packageContext=v1,interfacePrefix=I

Related issues/PRs

I have not found a related issue/PR.

Suggest a fix/enhancement

I would be interested very much to fix this problem but I was hoping to get a bit of feedback before I start with it (@jimschubert ?, @mandrean ?).

Especially the deserialization issue is not so easy. At the moment I'm considering two possible solutions but maybe there are even better ideas...:

1. Rely on json.net and the nancy integration. This seems to be the industry standard for c# and is also used by the Restsharp swagger client. However it would be an additional dependency. This was also suggested by the folks from the #nancyfx slack channel.
2. Build a custom ModelBinder for every type with a discriminator. This will probably be a bit slower than the json.net implementation but it would avoid the introduction of an additional dependency.

In order to solve the problem with the model builder, I suggest to use the approach suggested here.

Start with the NancyFx generator entry point invoked by -l nancyfx and inspect how the Pet/Cat definitions with allOf and a discriminator are handled. Compare generated builders, model binding, and serialization for the sample /pet endpoint. Done means inheritance is supported for model construction and subtype deserialization while preserving the existing serialization behavior.