feat(typescript-discriminator): use of discriminated types instead of a discriminator model

Created by: pdeszynski

BREAKING This code changes are only for TypeScript code generation.

This code changes the way types with discriminator fields are handled. Currently a "virtual" model is being returned (at leas based on typehints) which has all fields added from all discriminated types. Also this model does have attributes of a type that's last in the list.

This PR changes that the discriminated model will only have one field - discriminator and none other (of an union type of all subtypes discriminator field type). So in example, such schema:

{
  "openapi": "3.0.2",
  "info": {
    "title": "title",
    "version": "0.1.0"
  },
  "paths": {
    "/test": {
      "get": {
        "summary": "Test",
        "operationId": "test",
        "parameters": [],
        "responses": {
          "200": {
            "description": "Successful Response",
            "content": {
              "application/json": {
                "schema": {
                  "type": "array",
                  "items": {
                    "$ref": "#/components/schemas/DiscriminatedResult"
                  }
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "DiscriminatedResult": {
        "title": "DiscriminatedResult",
        "discriminator": {
          "propertyName": "type_name",
          "mapping": {
            "TypeFirst": "#/components/schemas/TypeFirst",
            "TypeSecond": "#/components/schemas/TypeSecond",
            "TypeThird": "#/components/schemas/TypeThird"
          }
        },
        "oneOf": [
          {
            "$ref": "#/components/schemas/TypeFirst"
          },
          {
            "$ref": "#/components/schemas/TypeSecond"
          },
          {
            "$ref": "#/components/schemas/TypeThird"
          }
        ]
      },
      "TypeFirst": {
        "title": "TypeFirst",
        "type": "object",
        "properties": {
          "additional_enum": {
            "title": "TypeFirst Enum",
            "default": "TypeFirst",
            "enum": ["TypeFirst"],
            "type": "string"
          },
          "value": {
            "title": "Value1",
            "type": "array",
            "items": {
              "type": "string"
            }
          },
          "type_name": {
            "title": "Type Name",
            "default": "TypeFirst",
            "enum": ["TypeFirst"],
            "type": "string"
          }
        },
        "required": ["value"]
      },
      "TypeSecond": {
        "title": "TypeSecond",
        "type": "object",
        "properties": {
          "additional_enum": {
            "title": "TypeSecond Enum",
            "default": "TypeSecond",
            "enum": ["TypeSecond"],
            "type": "string"
          },
          "value": {
            "title": "Value2",
            "type": "string",
            "items": {
              "type": "number"
            }
          },
          "type_name": {
            "title": "Type Name",
            "default": "TypeSecond",
            "enum": ["TypeSecond"],
            "type": "string"
          }
        },
        "required": ["value"]
      },
      "TypeThird": {
        "title": "TypeThird",
        "type": "object",
        "properties": {
          "additional_enum": {
            "title": "TypeThird Enum",
            "default": "TypeThird",
            "enum": ["TypeThird"],
            "type": "string"
          },
          "value": {
            "title": "Value",
            "type": "string",
            "items": {
              "type": "boolean"
            }
          },
          "type_name": {
            "title": "Type Name",
            "default": "TypeThird",
            "enum": ["TypeThird"],
            "type": "string"
          }
        },
        "required": ["value"]
      }
    }
  }
}

Will generate DiscriminatedResult:

export class DiscriminatedResult {
    'typeName': string;

    static discriminator: string | undefined = "typeName";

    static attributeTypeMap: Array<{name: string, baseName: string, type: string}> = [
        {
            "name": "typeName",
            "baseName": "type_name",
            "type": "string"
        }    ];

    static getAttributeTypeMap() {
        return DiscriminatedResult.attributeTypeMap;
    }
}

And API return type (body) will be of type: body: Array<TypeSecond | TypeFirst | TypeThird>; }

Additionally got rid of <any> typecasting in enums, so now TypeScript will correctly narrow types when discriminator is checked:

My question is if that's a correct approach, if yes I can make further changes for this PR (adding some tests / checking other typescript generators than node version)

Best

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo (2019/11) @amakhrov @davidgamero @mkusaka

PR checklist

• Read the contribution guidelines.
• [x Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.