Incorrect AJAX lookup URLs when parent app is not rooted at /

[vsemionov]

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

When your root (non-admin) app is rooted at /something, the admin app's AJAX lookup endpoint is at /something/admin/...
However, in the generated UI, the actual requests are sent to /admin/... and fail.
It's because the code that generates these URLs doesn't take into account any root path prefix:

view.ajax_lookup_url = urljoin(
    self.base_url + "/", f"{view.identity}/ajax/lookup"
)

Steps to reproduce the bug

No response

Expected behavior

No response

Actual behavior

No response

Debugging material

No response

Environment

MacOS, Python 3.13, SQLAdmin 0.23.0

Additional context

No response

[mmzeynalli]

@JartanFTW does #996 also fix (or can fix) this? If yes, could you please add tests as well?

[JartanFTW]

PR (#996) adds a RootPathMiddleware that fixes server-side routing and template URL generation when root_path is set, but it doesn't fully address this issue. The ajax_lookup_url is computed once at startup via urljoin(self.base_url, ...) and stored as a class variable — it never sees root_path. The middleware fixes routing if the request reaches the app, but the URL in the HTML data-url attribute is still wrong.
To properly fix this, ajax_lookup_url needs to be generated dynamically at render time using request.url_for("admin:ajax_lookup", identity=view.identity) instead of the pre-computed value in AjaxSelect2Widget.

[mmzeynalli]

Would you be willing to fix this in your PR? Or should we keep this separate?

[JartanFTW]

Would you be willing to fix this in your PR? Or should we keep this separate?

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