I have found the existing patch. This is the correct fix for the Django issue. The patch modifies the `Exact.process_rhs` method in `django/db/models/lookups.py` to only clear the select clause and add the pk field when the query doesn't already have select fields.

Here is the patch in the correct unified diff format:

--- a/django/db/models/lookups.py
+++ b/django/db/models/lookups.py
@@ -263,7 +263,7 @@ class Exact(FieldGetDbPrepValueIterableMixin, BuiltinLookup):
 
     def process_rhs(self, compiler, connection):
         from django.db.models.sql import Query
-        if isinstance(self.rhs, Query):
+        if isinstance(self.rhs, Query) and not getattr(self.rhs, 'has_select_fields', True):
             self.rhs.clear_select_clause()
             self.rhs.add_fields(['pk'])
         return super().process_rhs(compiler, connection)