Merge pull request #16400 from bogdan/last-with-sql

Always use SQL limit in Relation#last when limit argument given

activerecord/CHANGELOG.md

+*   Rework `ActiveRecord::Relation#last` 
+    
+    1. Always find last with ruby if relation is loaded
+    2. Always use SQL instead if relation is not loaded.
+    3. Deprecated relation loading when SQL order can not be automatically reversed
+
+        Topic.order("title").load.last(3)
+          # before: SELECT ...
+          # after: No SQL
+
+        Topic.order("title").last
+          # before: SELECT * FROM `topics`
+          # after:  SELECT * FROM `topics` ORDER BY `topics`.`title` DESC LIMIT 1
+
+        Topic.order("coalesce(author, title)").last
+          # before: SELECT * FROM `topics`
+          # after:  Deprecation Warning for irreversible order
+
+    *Bogdan Gusiev*
+
 *   `ActiveRecord::Relation#reverse_order` throws `ActiveRecord::IrreversibleOrderError`
     when the order can not be reversed using current trivial algorithm.
     Also raises the same error when `#reverse_order` is called on

activerecord/lib/active_record/relation/finder_methods.rb

@@ -145,15 +145,23 @@ def first!
     #
     #   [#<Person id:4>, #<Person id:3>, #<Person id:2>]
     def last(limit = nil)
-      if limit
-        if order_values.empty? && primary_key
-          order(arel_table[primary_key].desc).limit(limit).reverse
-        else
-          to_a.last(limit)
-        end
-      else
-        find_last
-      end
+      return find_last(limit) if loaded?
+
+      result = order_values.empty? && primary_key ? order(arel_table[primary_key].desc) : reverse_order
+      result = result.limit!(limit || 1)
+      limit ? result.reverse : result.first
+    rescue ActiveRecord::IrreversibleOrderError
+      ActiveSupport::Deprecation.warn(<<-WARNING.squish)
+          Finding a last element by loading the relation when SQL ORDER
+          can not be reversed is deprecated.
+          Rails 5.1 will raise ActiveRecord::IrreversibleOrderError in this case.
+          Please call `to_a.last` if you still want to load the relation.
+      WARNING
+      find_last(limit)
+    end
+
+    def find_last(limit)
+      limit ? to_a.last(limit) : to_a.last
     end
 
     # Same as #last but raises ActiveRecord::RecordNotFound if no record
@@ -523,19 +531,6 @@ def find_nth_with_limit(index, limit)
       relation.limit(limit).to_a
     end
 
-    def find_last
-      if loaded?
-        @records.last
-      else
-        @last ||=
-          if limit_value
-            to_a.last
-          else
-            reverse_order.limit(1).to_a.first
-          end
-      end
-    end
-
     private
 
     def find_nth_with_limit_and_offset(index, limit, offset:) # :nodoc:

activerecord/test/cases/finder_test.rb

@@ -506,7 +506,7 @@ def test_model_class_responds_to_last_bang
     end
   end
 
-  def test_take_and_first_and_last_with_integer_should_use_sql_limit
+  def test_take_and_first_and_last_with_integer_should_use_sql
     assert_sql(/LIMIT|ROWNUM <=/) { Topic.take(3).entries }
     assert_sql(/LIMIT|ROWNUM <=/) { Topic.first(2).entries }
     assert_sql(/LIMIT|ROWNUM <=/) { Topic.last(5).entries }
@@ -516,16 +516,30 @@ 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)
+  def test_last_with_integer_and_order_should_use_sql
+    relation = Topic.order("title")
+    assert_queries(1) { relation.last(5) }
+    assert !relation.loaded?
   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)
+  def test_last_with_integer_and_reorder_should_use_sql
+    relation = Topic.reorder("title")
+    assert_queries(1) { relation.last(5) }
+    assert !relation.loaded?
+  end
+
+  def test_last_on_loaded_relation_should_not_use_sql
+    relation  = Topic.limit(10).load
+    assert_no_queries do
+      relation.last
+      relation.last(2)
+    end
+  end
+
+  def test_last_with_irreversible_order
+    assert_deprecated do
+      Topic.order("coalesce(author_name, title)").last
+    end
   end
 
   def test_take_and_first_and_last_with_integer_should_return_an_array