Active Record Union Scopes

active_record_extension.rb

# from https://stackoverflow.com/questions/6686920/activerecord-query-union
# lib/active_record_extension.rb
module ActiveRecordExtension
  extend ActiveSupport::Concern

  class_methods do
    def union_scope(*scopes)
      id_column = "#{table_name}.#{primary_key}"
      sub_query = scopes.map { |s| s.select(id_column).to_sql }.join(" UNION ")
      where "#{id_column} IN (#{sub_query})"
    end
  end

  ActiveRecord::Base.send(:include, ActiveRecordExtension)

end

extensions.rb

# from https://stackoverflow.com/questions/6686920/activerecord-query-union
# config/initializers/extensions.rb
require "active_record_extension"

user.rb

# from https://stackoverflow.com/questions/6686920/activerecord-query-union
# example in some model
scope :active_nearby,     -> { where(active: true).where('distance <= 25') }
scope :inactive_distant,  -> { where(active: false).where('distance >= 200') }

scope :active_near_and_inactive_distant, -> { union_scope(active_nearby, inactive_distant) }

Less problems and easier to follow:

    def union_scope(*scopes)
      scopes.inject(self) { |all, scope| all.or(where(id: scope)) }
    end