ActiveRecord Validate Uniqueness with Scope Allowing Nil on Scope

Yes, with a Proc and :unless on the validator.

class Book < ActiveRecord::Base

  belongs_to :author
  validates :name, uniqueness: { scope: :author_id }, unless: Proc.new { |b| b.author_id.blank? }

end

Recommended Reading: http://guides.rubyonrails.org/active_record_validations.html#using-a-proc-with-if-and-unless

Make it conditional:

  validates :name, uniqueness: { scope: :author_id }, if: :author_id?