Based on my knowledge of Django and the issue description, I can construct the patch. The bug is in how `defer()` handles the case when it's chained after `only()`. The key file is `django/db/models/query.py` and the method `add_deferred_loading` in `django/db/models/sql/query.py`.

Looking at the issue and the discussion, the bug is specifically in the `add_deferred_loading` method. When `defer()` is called after `only()`, it needs to properly handle removing fields from the "only" set and potentially switching modes when all fields are removed.

The fix mentioned in the discussion is that when you call `defer()` after `only()`, and you're deferring fields that were in the `only()` set, you need to:
1. Remove those fields from the only set
2. But if there are additional fields being deferred that weren't in the only set, those should become deferred fields

Let me create the patch based on the Django source code structure:

--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -2105,12 +2105,15 @@ class Query(BaseExpression):
         When the existing set is "immediate", this will work out to
         removing any field names from the existing set.
         """
+        field_names = set(field_names)
         existing, defer = self.deferred_loading
         if defer:
             # Add to existing deferred names.
-            self.deferred_loading = existing.union(field_names), True
+            self.deferred_loading = existing.union(field_names), defer
         else:
-            # Remove names from the set of any existing "immediate load" names.
-            self.deferred_loading = existing.difference(field_names), False
+            # Remove names from the set of any existing "immediate load" names.
+            # If nothing remains, switch to defer mode.
+            new_existing = existing.difference(field_names)
+            new_defer = field_names.difference(existing)
+            if new_existing:
+                self.deferred_loading = new_existing, False
+            else:
+                self.deferred_loading = new_defer, True