Extends the functionality set forth in union_scope.rb, to include EXCEPT and INTERSECT operations

set_operations.rb

module ActiveRecord
  module Scopes
    module SetOperations
      extend ActiveSupport::Concern

      class_methods do

        def union_scope(*scopes)
          apply_operation 'UNION', scopes
        end

        def intersect_scope(*scopes)
          apply_operation 'INTERSECT', scopes
        end

        def except_scope(*scopes)
          apply_operation 'EXCEPT', scopes
        end

        private

        def apply_operation(operation, scopes)
          id_column = "#{table_name}.#{primary_key}"
          sub_query = scopes
                      .map { |s| s.select(id_column).to_sql }
                      .join(" #{operation} ")

          where "#{id_column} IN (#{sub_query})"
        end
      end
    end
  end
end

Less problems and easier to follow:

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