3 files changed, 24 insertions, 1 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 4a9022e66b..43651c9b48 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Fix validation on uniqueness of empty association.
+
+    *Evgeny Li*
+
 *   Make `ActiveRecord::Relation#unscope` affect relations it is merged in to.
 
     *Jon Leighton*
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index b55af692d6..38f37f5c8a 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -48,7 +48,7 @@ module ActiveRecord
       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]
diff --git a/activerecord/test/cases/validations/uniqueness_validation_test.rb b/activerecord/test/cases/validations/uniqueness_validation_test.rb
index 56d8db0be9..71cb9d7b1e 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/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 @@ class UniquenessValidationTest < ActiveRecord::TestCase
       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