allow an association as a scope parameter

#4321

2 files changed, 17 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 55edb57206..77ddd72e59 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -25,6 +25,11 @@ module ActiveRecord
 
         Array(options[:scope]).each do |scope_item|
           scope_value = record.send(scope_item)
+          reflection = record.class.reflect_on_association(scope_item)
+          if reflection
+            scope_value = record.send(reflection.foreign_key)
+            scope_item = reflection.foreign_key
+          end
           relation = relation.and(table[scope_item].eq(scope_value))
         end
 
diff --git a/activerecord/test/cases/validations/uniqueness_validation_test.rb b/activerecord/test/cases/validations/uniqueness_validation_test.rb
index 327830096b..44c2812bff 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/activerecord/test/cases/validations/uniqueness_validation_test.rb
@@ -92,6 +92,18 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     assert r3.valid?, "Saving r3"
   end
 
+  def test_validate_uniqueness_with_object_scope
+    Reply.validates_uniqueness_of(:content, :scope => :topic)
+
+    t = Topic.create("title" => "I'm unique!")
+
+    r1 = t.replies.create "title" => "r1", "content" => "hello world"
+    assert r1.valid?, "Saving r1"
+
+    r2 = t.replies.create "title" => "r2", "content" => "hello world"
+    assert !r2.valid?, "Saving r2 first time"
+  end
+
   def test_validate_uniqueness_scoped_to_defining_class
     t = Topic.create("title" => "What, me worry?")