Showcase the usage of callback of the middleware

Performing user onboarding - The callback sample checks for user's existence and creates if there is no such user with the given identity.

Author: ArtyomVancyan

examples/demonstration/main.py

@@ -1,18 +1,38 @@
 from fastapi import APIRouter
 from fastapi import FastAPI
+from sqlalchemy.orm import Session
 
 from config import oauth2_config
 from database import Base
 from database import engine
+from database import get_db
 from fastapi_oauth2.middleware import OAuth2Middleware
+from fastapi_oauth2.middleware import User
 from fastapi_oauth2.router import router as oauth2_router
+from models import User as UserModel
 from router import router as app_router
 
 Base.metadata.create_all(bind=engine)
 
 router = APIRouter()
 
+
+async def on_auth(user: User):
+    # perform a check for user existence in
+    # the database and create if not exists
+    db: Session = next(get_db())
+    query = db.query(UserModel)
+    if not query.filter_by(identity=user.identity).first():
+        UserModel(**{
+            "identity": user.get("identity"),
+            "username": user.get("username"),
+            "image": user.get("image"),
+            "email": user.get("email"),
+            "name": user.get("name"),
+        }).save(db)
+
+
 app = FastAPI()
 app.include_router(app_router)
 app.include_router(oauth2_router)
-app.add_middleware(OAuth2Middleware, config=oauth2_config)
+app.add_middleware(OAuth2Middleware, config=oauth2_config, callback=on_auth)