How do I format and style a sub table in Prawn?

I'm new to generating PDFs with Prawn so this may be a simple question but it's driving me crazy!

I have a table with a nested sub table. I have been able to style and format the main table easily but I can't quite seem to do the same with the nested sub table. All I really need to do to it is set the column widths and remove the borders but I can't seem to figure it out.

Here's the code I have so far:

  def line_items
    data = line_item_rows
    table(data)  do
       row(0).font_style = :bold
       columns(0).width = 160
       columns(1).width = 300
       columns(2).align = :right
       columns(2).valign = :bottom
       row(0).columns(2).valign = :top
       row(0).columns(2).align = :left
       self.header = true
    end   
  end

  def line_item_rows
    [["Description", "Items" ,"Price ex GST"]] +
    @line_items.map do |item|
      [item.description, sub_item_rows(item), price(item.charge_ex_gst)] 
    end +
    [["","Total", price(@project.charge_ex_gst)]]
  end

  def sub_item_rows(item)
   item.sub_items.map do |sub_item|
      ["#{sub_item.quantity} x  #{sub_item.name} #{price(sub_item.total_charge_ex_gst)}"] 
    end
  end

Any suggestions as to to how I apply styling to the sub table? Thanks in advance for your help.

Cheers, Mark

OK, here's how it was fixed. I needed to use "make_table" and apply the formatting there like so:

  def line_items
    move_down 15
    data = line_item_rows

        table(data) do
           row(0).font_style = :bold
           columns(0).width = 160
           columns(1).width = 300
           columns(2).align = :right
           columns(2).valign = :bottom
           row(0).columns(2).valign = :top
           row(0).columns(2).align = :left
           self.header = true
        end     
  end

  def line_item_rows
    [["Description", "Items" ,"Price ex GST"]] +
    @line_items.map do |item|
      [item.description, 
        sub_items(item), 
        price(item.charge_ex_gst)] 
    end +
    [["","Grand Total", price(@project.charge_ex_gst)]]
  end

  def sub_items(item)
    sub_data = sub_item_rows(item)
    make_table(sub_data) do
       columns(0).width = 200
       columns(1).width = 100
       columns(1).align = :right
       #columns(0).borders = []
    end   
  end

  def sub_item_rows(item)
   item.sub_items.map do |sub_item|
      ["#{sub_item.quantity} x #{sub_item.name}", price(sub_item.total_charge_ex_gst)] 
    end  +
      [["","Total"]]
  end