diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 9fd8b49ffb3c10ff389694fb9e7e388efcbc53d7..4a41f1ea8bd43790621ba8da7c05f0547f0c5a61 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -87,6 +87,9 @@ def first!
       first or raise RecordNotFound
     end
 
+    # Find the last record (or last N records if a parameter is supplied).
+    # If no order is defined it will order by primary key.
+    #
     # Examples:
     #
     #   Person.last # returns the last object fetched by SELECT * FROM people
@@ -94,8 +97,8 @@ def first!
     #   Person.order("created_on DESC").offset(5).last
     def last(limit = nil)
       if limit
-        if order_values.empty?
-          order("#{primary_key} DESC").limit(limit).reverse
+        if order_values.empty? && primary_key
+          order("#{quoted_table_name}.#{quoted_primary_key} DESC").limit(limit).reverse
         else
           to_a.last(limit)
         end