How can show the next value from a value saved in a table?

Question

I'm trying to create an initial number when the user saves the initial number of an invoice will continue with next value

Table example working:

|invoice_nums|
  |id|   |invoice_num|
   1        3010
|invoices|
  |id|   |invoice_num| |amount|
   1        3011         1000
   2        3012         1000
   3        3013         1000 

Table example not working when i change the invoice_num value from InvoiceNums:

|invoice_nums|
  |id|   |invoice_num|
   1        2010
|invoices|
  |id|   |invoice_num| |amount|
   4        2014         1000      ###SHOULD BE 2011
   5        2015         1000      ###SHOULD BE 2012 

Here is my controller:

  def new
   @num_ini = InvoiceNum.first
   @num_last = Invoice.last
   @trick = Invoice.count(:all) 
  end

Here is my view:

<% @sum= "%06d" % (@num.invoice_num.next.to_i + @trick.to_i) %>
<%= @sum %>

I tried this in the view

<%= @sum= (@num_last.invoice_num.to_i  || @num_ini.invoice_num.to_i )+ 1   %>

Also I tried this in the view:

<% if @num_ini.invoice_num.to_i > @num_last.invoice_num.to_i   %>
    <%= @sum= @num_ini.invoice_num.to_i + 1 %>
<% else %>
    <%= @sum=@num_last.invoice_num.to_i + 1 %>
<% end %>

Please somebody can help me?

I'm trying to continue the invoice_num according the last invoice_num from the invoice_nums.

All help can be accepted.

Answer 1

class Invoice < ActiveRecord::Base
  INITIAL_INVOICE_NUMBER = 2009
  before_create :set_invoice_number

  def set_invoice_number
    (Invoice.maximum(:invoice_number) || self::INITIAL_INVOICE_NUMBER) + 1
  end

end

Note:

I made the initial 2009 because of the +1 either that or change the line to a trinary condition