Fix validation on uniqueness of empty association

activerecord/CHANGELOG.md

+*   Fix validation on uniqueness of empty association.
+
+    *Evgeny Li*
+
 *   Make `ActiveRecord::Relation#unscope` affect relations it is merged in to.
 
     *Jon Leighton*

activerecord/lib/active_record/validations/uniqueness.rb

@@ -48,7 +48,7 @@ def find_finder_class_for(record) #:nodoc:
       def build_relation(klass, table, attribute, value) #:nodoc:
         if reflection = klass.reflect_on_association(attribute)
           attribute = reflection.foreign_key
-          value = value.attributes[reflection.primary_key_column.name]
+          value = value.attributes[reflection.primary_key_column.name] unless value.nil?
         end
 
         column = klass.columns_hash[attribute.to_s]

activerecord/test/cases/validations/uniqueness_validation_test.rb

@@ -35,6 +35,11 @@ class Employee < ActiveRecord::Base
   validates_uniqueness_of :nicknames
 end
 
+class TopicWithUniqEvent < Topic
+  belongs_to :event, foreign_key: :parent_id
+  validates :event, uniqueness: true
+end
+
 class UniquenessValidationTest < ActiveRecord::TestCase
   fixtures :topics, 'warehouse-things', :developers
 
@@ -376,4 +381,18 @@ def test_validate_uniqueness_with_array_column
       assert_equal ["has already been taken"], e2.errors[:nicknames], "Should have uniqueness message for nicknames"
     end
   end
+
+  def test_validate_uniqueness_on_existing_relation
+    event = Event.create
+    assert TopicWithUniqEvent.create(event: event).valid?
+
+    topic = TopicWithUniqEvent.new(event: event)
+    assert_not topic.valid?
+    assert_equal ['has already been taken'], topic.errors[:event]
+  end
+
+  def test_validate_uniqueness_on_empty_relation
+    topic = TopicWithUniqEvent.new
+    assert topic.valid?
+  end
 end