allow association as 1st uniqueness validation arg

#4321

activerecord/lib/active_record/validations/uniqueness.rb

@@ -56,7 +56,15 @@ module ActiveRecord
       end
 
       def build_relation(klass, table, attribute, value) #:nodoc:
+        reflection = klass.reflect_on_association(attribute)
+        column = nil
+        if(reflection)
+          column = klass.columns_hash[reflection.foreign_key]
+          attribute = reflection.foreign_key
+          value = value.attributes[reflection.primary_key_column.name]
+        else
           column = klass.columns_hash[attribute.to_s]
+        end
         value = column.limit ? value.to_s[0, column.limit] : value.to_s if !value.nil? && column.text?
 
         if !options[:case_sensitive] && value && column.text?

activerecord/test/cases/validations/uniqueness_validation_test.rb

@@ -104,6 +104,18 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     assert !r2.valid?, "Saving r2 first time"
   end
 
+  def test_validate_uniqueness_with_object_arg
+    Reply.validates_uniqueness_of(: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?")