1 files changed, 22 insertions, 25 deletions
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index a4280c5f33..0037398554 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -145,15 +145,21 @@ module ActiveRecord
     #
     #   [#<Person id:4>, #<Person id:3>, #<Person id:2>]
     def last(limit = nil)
-      if limit
-        if order_values.empty? && primary_key
-          order(arel_attribute(primary_key).desc).limit(limit).reverse
-        else
-          to_a.last(limit)
-        end
-      else
-        find_last
-      end
+      return find_last(limit) if loaded? || limit_value
+
+      result = limit(limit || 1)
+      result.order!(arel_attribute(primary_key)) if order_values.empty? && primary_key
+      result = result.reverse_order!
+
+      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
 
     # Same as #last but raises ActiveRecord::RecordNotFound if no record
@@ -330,7 +336,7 @@ module ActiveRecord
     end
 
     # This method is called whenever no records are found with either a single
-    # id or multiple ids and raises a ActiveRecord::RecordNotFound exception.
+    # id or multiple ids and raises an ActiveRecord::RecordNotFound exception.
     #
     # The error message is different depending on whether a single id or
     # multiple ids are provided. If multiple ids are provided, then the number
@@ -500,7 +506,7 @@ module ActiveRecord
     def find_some_ordered(ids)
       ids = ids.slice(offset_value || 0, limit_value || ids.size) || []
 
-      result = except(:limit, :offset).where(primary_key => ids).to_a
+      result = except(:limit, :offset).where(primary_key => ids).records
 
       if result.size == ids.size
         pk_type = @klass.type_for_attribute(primary_key)
@@ -516,7 +522,7 @@ module ActiveRecord
       if loaded?
         @records.first
       else
-        @take ||= limit(1).to_a.first
+        @take ||= limit(1).records.first
       end
     end
 
@@ -555,19 +561,6 @@ module ActiveRecord
       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:
@@ -578,5 +571,9 @@ module ActiveRecord
         find_nth_with_limit(index, limit)
       end
     end
+
+    def find_last(limit)
+      limit ? records.last(limit) : records.last
+    end
   end
 end