rails/rails — pull request #12774 by exAspArk (2013-11-05 14:56:17+00:00)
Title: Fix validation on uniqueness

In case if I set validation on uniqueness of relation
```ruby
class Foo < ActiveRecord::Base
belongs_to :bar
validates :bar, uniqueness: true
end
```

and it doesn't exist, I have an error:
```
NoMethodError - undefined method `attributes' for nil:NilClass:
activerecord (4.0.1.rc4) lib/active_record/validations/uniqueness.rb:56:in `build_relation'
```

To fix it I suggest adding the condition