nil_user.rb

class NilUser
  def display_name
    "Uknown user"
  end
end

post.rb

class Post < ActiveRecord::Base
  belongs_to: user, nil_class: NilUser
end

class NilUser
  def display_name
    "Uknown user"
  end
end

class Post < ActiveRecord::Base
  belongs_to: user

  # Can add some instance variable caching if necessary.
  def user
    super || NilUser.new
  end
end

This looks more clear and intention revealing to me.

@carlosantoniodasilva I know it can be done this way, but it's too verbose to do it for every class, IMHO this is a better option

class Post < ActiveRecord::Base
  belongs_to :user, nil_class: NilUser
end

I would actually go even further then that (needs to be accepted by core members of course)

class Post < ActiveRecord::Base
  belongs_to :user #Will try to load NilUser.new if nil automatically

From what I have seen done in rails over the years is that the first option is done first and the second later.

imo you've just overlooked the purpose of the null object pattern and/or the availability of #try in rails.

class Post < ActiveRecord::Base
  belongs_to :user

  def author
    user.try(:name)
  end
end