Ajax query passes incorrect urn when admin mounted on sub fastapi app

[infanasotku]

Checklist

• The bug is reproducible against the latest release or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

Ajax request from browser doesn't consider urn which setted up by sub fastapi app.

Steps to reproduce the bug

1. Link admin to sub fastapi app:

from fastapi import FastApi
from sqladmin import Admin

app = FastApi()
subapp = FastApi()
app.mount('/suburn', subapp)

admin = Admin(subapp, base_url='/') 

2. Link any ModelView with relationship with admin which forces ajax request at select list on create page.
3. Click on select list on create page and try to type symbols.

Expected behavior

Ajax should request to uri as: .../suburn/ModelViewName/ajax/...

Actual behavior

Ajax request to uri: .../ModelViewName/ajax/... with skipping suburn.

P.S. Admin page routing is correct with sub app.

Debugging material

In my way:
suburn='/admin'
base_url='/'

[INFO] uvicorn.access: 127.0.0.1:64082 - "GET /admin/enterprise/list HTTP/1.1" 200
[INFO] uvicorn.access: 127.0.0.1:64082 - "GET /admin/enterprise/create HTTP/1.1" 200
[INFO] uvicorn.access: 127.0.0.1:64082 - "GET /enterprise/ajax/lookup?name=company&term=2 HTTP/1.1" 404

Environment

OS:
MacOS 14.4.1

Python:
3.11.8

sqladmin==0.16.1
fastapi==0.110.1

Additional context

I understand that suburn not passed to template ajax, but all another routing work correctly - it confused me.
(I think that we need additional param like sub_url separatly off base_url to specific behavior.)

[IBestuzhev]

I encountered a very similar issue: we use FastAPI.root_path parameter because app is running behind a proxy.

I think the most correct way is to rely on FastAPI resolver - call fastapi.Request.url_for.

Meanwhile, I did a quick workaround in my project:

        admin = Admin(
            app,
            engine,
            authentication_backend=auth_backend,
            base_url='/admin-ui',
        )
        admin.add_view(view)
        if view.is_model and app.root_path:
            # This URL goes directly into template into Select2 widget
            # With current code (sqladmin 0.16.1) it goes as is
            # The URL will be: /admin-ui/my-model/ajax/lookup
            # But the correct one for production is prefixed
            # /proxy-prefix/admin-ui/my-model/ajax/lookup
            view.ajax_lookup_url = app.root_path + view.ajax_lookup_url

[infanasotku]

@IBestuzhev yes, it works.

[aminalaee]

@IBestuzhev But the Request.url_for is not available in this context.

[mmzeynalli]

@JartanFTW Im wondering if #996 also fixes this?

[JartanFTW]

PR (#996) adds a RootPathMiddleware that addresses the root_path-behind-a-reverse-proxy variant of this issue, but not the original sub-app mounting scenario. When using app.mount('/suburn', subapp) with Admin(subapp, ...), the middleware ends up on the sub-app where it's effectively a no-op — it can't inject the missing mount prefix into the pre-computed ajax_lookup_url.

The underlying issue is the same as #1012: ajax_lookup_url is computed once at startup and doesn't account for mount prefixes. The fix is to generate it dynamically per-request via request.url_for() in the template/widget.