Don't remove join dependencies in `Relation#exists?`

Fixes #18632

activerecord/CHANGELOG.md

+*   Don't remove join dependencies in `Relation#exists?`
+
+    Fixes #18632
+
+    *Sean Griffin*
+
 *   Invalid values assigned to a JSON column are assumed to be `nil`.
 
     Fixes #18629.

activerecord/lib/active_record/relation/finder_methods.rb

@@ -307,7 +307,7 @@ def exists?(conditions = :none)
         relation = relation.where(conditions)
       else
         unless conditions == :none
-          relation = where(primary_key => conditions)
+          relation = relation.where(primary_key => conditions)
         end
       end
 

activerecord/test/cases/relations_test.rb

@@ -857,6 +857,12 @@ def test_exists
     assert ! fake.exists?(authors(:david).id)
   end
 
+  def test_exists_uses_existing_scope
+    post = authors(:david).posts.first
+    authors = Author.includes(:posts).where(name: "David", posts: { id: post.id })
+    assert authors.exists?(authors(:david).id)
+  end
+
   def test_last
     authors = Author.all
     assert_equal authors(:bob), authors.last