enumerator.rb

class MyEnumerator
  attr_reader :scope, :block, :opened, :headers

  def initialize(scope, headers, &block)
    @scope = scope
    @block = block
    @opened = false
    @cursor = PostgreSQLCursor::Cursor.new(scope.send(:to_unprepared_sql))
    @headers = headers
    @headers_provided = false
  end

  def next
    open unless opened

    unless @headers_provided
      @headers_provided = true
      return headers
    end

    row = fetch_row

    if row.nil?
      close
      raise StopIteration
    end

    return @block.call(row)
  end

  def fetch_row
    @column_types ||= @cursor.column_types

    if row = @cursor.fetch
      scope.klass.send(:instantiate, row, @column_types)
    else
      nil
    end
  end

  def open
    @cursor.open
    @opened = true
  end

  def close
    @cursor.close
    @opened = false
  end
end

usage.rb

MyEnumerator.new(scope, csv_header) do |revenue|
  next if @options[:ignore_zero] && revenue.chain.any?(&:zero?)

  CSV.generate_line(row(revenue))
end