any_of.rb

module AnyOf # makes "OR" queries
  def any_of *queries
    queries.inject(arelify queries.pop) do |these, query|
      these.or arelify query
    end
  end

  private

  def arelify query # tempting to move this into a "to_arel" method
    ActiveRecord::Relation === query && query.arel.ast.cores.last.wheres || query
  end
end

example.rb

class User < ActiveRecord::Base
  extend AnyOf
  
  scope(:assignable) { where role: 0   }
  scope(:unknown)    { where role: nil }
  
  scope(:unroled){ where any_of(assignable, unknown) }
  # SQL:  "SELECT `users`.* FROM `users`  WHERE ((`users`.`id` IS NULL OR `users`.`id` = 0))"
end