Use LIMIT sql word in last when it's possible

2 files changed, 23 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 2639c46709..83d650d3f4 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -134,7 +134,11 @@ module ActiveRecord
     def last(*args)
       if args.any?
         if args.first.kind_of?(Integer) || (loaded? && !args.first.kind_of?(Hash))
-          to_a.last(*args)
+          if order_values.empty? && reorder_value.nil?
+            order("#{primary_key} DESC").limit(*args).reverse
+          else
+            to_a.last(*args)
+          end
         else
           apply_finder_options(args.first).last
         end
diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index fbcae27719..d840d38678 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -243,8 +243,25 @@ class FinderTest < ActiveRecord::TestCase
     end
   end
 
-  def test_first_with_integer_should_use_sql_limit
+  def test_first_and_last_with_integer_should_use_sql_limit
     assert_sql(/LIMIT 2/) { Topic.first(2).entries }
+    assert_sql(/LIMIT 5/) { Topic.last(5).entries }
+  end
+
+  def test_last_with_integer_and_order_should_keep_the_order
+    assert_equal Topic.order("title").to_a.last(2), Topic.order("title").last(2)
+  end
+
+  def test_last_with_integer_and_order_should_not_use_sql_limit
+    query = assert_sql { Topic.order("title").last(5).entries }
+    assert_equal 1, query.length
+    assert_no_match(/LIMIT/, query.first)
+  end
+
+  def test_last_with_integer_and_reorder_should_not_use_sql_limit
+    query = assert_sql { Topic.reorder("title").last(5).entries }
+    assert_equal 1, query.length
+    assert_no_match(/LIMIT/, query.first)
   end
 
   def test_first_and_last_with_integer_should_return_an_array