Add support for resource inheritance in OpenAPI

docs/docfx.json

@@ -23,6 +23,7 @@
         "files": [
           "api/**.yml",
           "api/index.md",
+          "ext/openapi/index.md",
           "getting-started/**.md",
           "getting-started/**/toc.yml",
           "usage/**.md",

docs/ext/openapi/index.md

@@ -0,0 +1,127 @@
+# JSON:API Extension for OpenAPI
+
+This extension facilitates using OpenAPI client generators targeting JSON:API documents.
+
+In JSON:API, a resource object contains the `type` member, which defines the structure of nested [attributes](https://jsonapi.org/format/#document-resource-object-attributes) and [relationships](https://jsonapi.org/format/#document-resource-object-relationships) objects.
+While OpenAPI supports such constraints using `allOf` inheritance with a discriminator property for the `data` member,
+it provides no way to express that the discriminator recursively applies to nested objects.
+
+This extension addresses that limitation by defining additional discriminator properties to guide code generation tools.
+
+## URI
+
+This extension has the URI `https://www.jsonapi.net/ext/openapi`.
+Because code generators often choke on the double quotes in `Accept` and `Content-Type` HTTP header values, a relaxed form is also permitted: `openapi`.
+
+For example, the following `Content-Type` header:
+
+```http
+Content-Type: application/vnd.api+json; ext="https://www.jsonapi.net/ext/openapi"
+```
+
+is equivalent to:
+
+```http
+Content-Type: application/vnd.api+json; ext=openapi
+```
+
+To avoid the need for double quotes when multiple extensions are used, the following relaxed form can be used:
+
+```http
+Content-Type: application/vnd.api+json; ext=openapi; ext=atomic
+```
+
+> [!NOTE]
+> The [base specification](https://jsonapi.org/format/#media-type-parameter-rules) *forbids* the use of multiple `ext` parameters
+> and *requires* that each extension name must be a URI.
+> This extension relaxes both constraints for practical reasons, to workaround bugs in client generators that produce broken code otherwise.
+
+## Namespace
+
+This extension uses the namespace `openapi`.
+
+> [!NOTE]
+> JSON:API extensions can only introduce new document members using a reserved namespace as a prefix.
+
+## Document Structure
+
+A document that supports this extension MAY include any of the top-level members allowed by the base specification,
+including any members defined in the [Atomic Operations extension](https://jsonapi.org/ext/atomic/).
+
+### Resource Objects
+
+In addition to the members allowed by the base specification, the following member MAY be included
+in [attributes](https://jsonapi.org/format/#document-resource-object-attributes) and [relationships](https://jsonapi.org/format/#document-resource-object-relationships) objects:
+
+* `openapi:discriminator` - A string that MUST be identical to the `type` member in the containing [resource object](https://jsonapi.org/format/#document-resource-objects).
+
+Here's how an article (i.e. a resource of type "articles") might appear in a document:
+
+```json
+{
+  "data": {
+    "type": "articles",
+    "id": "1",
+    "attributes": {
+      "openapi:discriminator": "articles",
+      "title": "Rails is Omakase"
+    },
+    "relationships": {
+      "openapi:discriminator": "articles",
+      "author": {
+        "data": { "type": "people", "id": "9" }
+      }
+    }
+  }
+}
+```
+
+### Atomic Operations
+
+In addition to the members allowed by the [Atomic Operations extension](https://jsonapi.org/ext/atomic/), 
+the following member MAY be included in elements of an `atomic:operations` array:
+
+* `openapi:discriminator` - A free-format string to facilitate generation of client code.
+
+For example:
+
+```http
+POST /operations HTTP/1.1
+Host: example.org
+Content-Type: application/vnd.api+json; ext="https://www.jsonapi.net/ext/openapi https://jsonapi.org/ext/atomic"
+Accept: application/vnd.api+json; ext="https://www.jsonapi.net/ext/openapi https://jsonapi.org/ext/atomic"
+
+{
+  "atomic:operations": [{
+    "openapi:discriminator": "add-article",
+    "op": "add",
+    "data": {
+      "type": "articles",
+      "attributes": {
+        "openapi:discriminator": "articles",
+        "title": "JSON API paints my bikeshed!"
+      }
+    }
+  }]
+}
+```
+
+## Processing
+
+A server MAY ignore the `openapi:discriminator` member in [attributes](https://jsonapi.org/format/#document-resource-object-attributes) and [relationships](https://jsonapi.org/format/#document-resource-object-relationships) objects from incoming requests.
+A server SHOULD ignore the `openapi:discriminator` member in elements of an `atomic:operations` array.
+
+A server MUST include the `openapi:discriminator` member in [attributes](https://jsonapi.org/format/#document-resource-object-attributes) and [relationships](https://jsonapi.org/format/#document-resource-object-relationships) objects in outgoing responses.
+The member value MUST be the same as the `type` member value of the containing resource object.
+
+A client MAY include the `openapi:discriminator` member in [attributes](https://jsonapi.org/format/#document-resource-object-attributes) and [relationships](https://jsonapi.org/format/#document-resource-object-relationships) objects in outgoing requests.
+The member value MUST be the same as the `type` member value of the containing resource object.
+
+A client MAY include the `openapi:discriminator` member in elements of an `atomic:operations` array.
+
+### Processing Errors
+
+A server SHOULD validate that the value of the `openapi:discriminator` member in
+[attributes](https://jsonapi.org/format/#document-resource-object-attributes) and [relationships](https://jsonapi.org/format/#document-resource-object-relationships) objects
+is identical to the `type` member in the containing resource object. When validation fails, the server MUST respond with a `409 Conflict`
+and SHOULD include a document with a top-level `errors` member that contains an error object.