Allow adding routers by module path string

docs/docs/guides/routers.md

@@ -109,17 +109,15 @@ api = NinjaAPI()
 
 Now we import all the routers from the various apps, and include them into the main API instance:
 
-```Python hl_lines="2 3 4 8 9 10"
+```Python hl_lines="2 6 7 8"
 from ninja import NinjaAPI
 from events.api import router as events_router
-from news.api import router as news_router
-from blogs.api import router as blogs_router
 
 api = NinjaAPI()
 
-api.add_router("/events/", events_router)
-api.add_router("/news/", news_router)
-api.add_router("/blogs/", blogs_router)
+api.add_router("/events/", events_router)    # You can add router object
+api.add_router("/news/", "news.api.router")  #   or well add router by python path
+api.add_router("/blogs/", "blogs.api.router")
 ```
 
 Now, include `api` to your urls as usual and open your browser at `/api/docs`, and you should see all your routers combined into a single API:

ninja/main.py

@@ -15,6 +15,7 @@
 
 from django.http import HttpRequest, HttpResponse
 from django.urls import URLPattern, URLResolver, reverse
+from django.utils.module_loading import import_string
 
 from ninja.constants import NOT_SET, NOT_SET_TYPE
 from ninja.errors import ConfigError, set_default_exc_handlers
@@ -359,12 +360,16 @@ def api_operation(
     def add_router(
         self,
         prefix: str,
-        router: Router,
+        router: Union[Router, str],
         *,
         auth: Any = NOT_SET,
         tags: Optional[List[str]] = None,
         parent_router: Router = None,
     ) -> None:
+        if isinstance(router, str):
+            router = import_string(router)
+            assert isinstance(router, Router)
+
         if auth is not NOT_SET:
             router.auth = auth
         if tags is not None:

tests/demo_project/demo/urls.py

@@ -1,12 +1,10 @@
 from django.contrib import admin
 from django.urls import path
-from multi_param.api import router as multi_param
-from someapp.api import router
 
 from ninja import NinjaAPI
 
 api_v1 = NinjaAPI()
-api_v1.add_router("events", router)
+api_v1.add_router("events", "someapp.api.router")
 # TODO: check ^ for possible mistakes like `/events` `events/``
 
 
@@ -42,7 +40,7 @@ def put_foobar(request):
 
 
 api_multi_param = NinjaAPI(version="1.0.1")
-api_multi_param.add_router("", multi_param)
+api_multi_param.add_router("", "multi_param.api.router")
 
 urlpatterns = [
     path("admin/", admin.site.urls),