SQL result set operations for ActiveRecord

set_operations.rb

module SetOperations
  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