having this model:

belongs_to :contact, autosave: true
validates_presence_of :contact

In theory, because it has autosave: true it should validate associated models before save. But I am forced to add this line if I want this nested validation:

validates_associated :contact

Better explanation here:
http://stackoverflow.com/questions/31469886/having-a-rails-model-with-autosave-should-i-validate-associated