Fix for #371

if a query contains a limit or an offset, ActiveRecord::FinderMethods#find_last had inconsistent behavior.

If the records were loaded, it returned the last record in the cached list.
If they were not, it reversed the order of the query and changed the limit to one.
If the earlier limit was less than the total matching the query in the db,
it would return a different record than if the records had been cached.

This commit changes find_last so that it loads the records when getting the
last record on a query containing a limit or an offset, which makes the behavior consistent.

activerecord/lib/active_record/relation/finder_methods.rb

@@ -375,7 +375,12 @@ module ActiveRecord
       if loaded?
         @records.last
       else
-        @last ||= reverse_order.limit(1).to_a[0]
+        @last ||=
+          if offset_value || limit_value
+            to_a.last
+          else
+            reverse_order.limit(1).to_a[0]
+          end
       end
     end
 

activerecord/test/cases/finder_test.rb

@@ -683,6 +683,27 @@ class FinderTest < ActiveRecord::TestCase
     assert_nil Topic.find_last_by_title_and_author_name(topic.title, "Anonymous")
   end
 
+  def test_find_last_with_limit_gives_same_result_when_loaded_and_unloaded
+    scope = Topic.limit(2)
+    unloaded_last = scope.last
+    loaded_last = scope.all.last
+    assert_equal loaded_last, unloaded_last
+  end
+
+  def test_find_last_with_limit_and_offset_gives_same_result_when_loaded_and_unloaded
+    scope = Topic.offset(2).limit(2)
+    unloaded_last = scope.last
+    loaded_last = scope.all.last
+    assert_equal loaded_last, unloaded_last
+  end
+
+  def test_find_last_with_offset_gives_same_result_when_loaded_and_unloaded
+    scope = Topic.offset(3)
+    unloaded_last = scope.last
+    loaded_last = scope.all.last
+    assert_equal loaded_last, unloaded_last
+  end
+
   def test_find_all_by_one_attribute
     topics = Topic.find_all_by_content("Have a nice day")
     assert_equal 2, topics.size