Add sub-app to generate open api spec

demo/__main__.py

@@ -0,0 +1,25 @@
+from aiohttp import web
+
+from aiohttp_pydantic import oas
+from aiohttp.web import middleware
+
+from .view import PetItemView, PetCollectionView
+from .model import Model
+
+
+@middleware
+async def pet_not_found_to_404(request, handler):
+    try:
+        return await handler(request)
+    except Model.NotFound as key:
+        return web.json_response({"error": f"Pet {key} does not exist"}, status=404)
+
+
+app = web.Application(middlewares=[pet_not_found_to_404])
+oas.setup(app)
+
+app["model"] = Model()
+app.router.add_view("/pets", PetCollectionView)
+app.router.add_view("/pets/{id}", PetItemView)
+
+web.run_app(app)