break cache if we're inside a "scoping" call. fixes #17052

For now, we don't want to take "scoping" calls in to account when
calculating cache keys for relations, so just opt-out.

activerecord/lib/active_record/associations/singular_association.rb

@@ -39,7 +39,12 @@ module ActiveRecord
         end
 
         def get_records
-          return scope.limit(1).to_a if reflection.scope_chain.any?(&:any?) || scope.eager_loading?
+          if reflection.scope_chain.any?(&:any?) ||
+              scope.eager_loading? ||
+              klass.current_scope
+
+            return scope.limit(1).to_a
+          end
 
           conn = klass.connection
           sc = reflection.association_scope_cache(conn, owner) do

activerecord/test/cases/scoping/relation_scoping_test.rb

@@ -15,6 +15,16 @@ class RelationScopingTest < ActiveRecord::TestCase
     developers(:david)
   end
 
+  def test_unscoped_breaks_caching
+    author = authors :mary
+    assert_nil author.first_post
+    post = FirstPost.unscoped do
+      author = authors :mary
+      author.reload.first_post
+    end
+    assert post
+  end
+
   def test_reverse_order
     assert_equal Developer.order("id DESC").to_a.reverse, Developer.order("id DESC").reverse_order
   end