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Editorial Team
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Asked: 2026-05-30T22:23:08+00:00

I’m a rails beginner (I think I have the basics down pretty well at

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I’m a rails beginner (I think I have the basics down pretty well at this point), and I’m working on an app with a simple parent-child structure: Company => Client => Group => Order. The relevant DB tables for this question are Group and Order (Group has_many Orders).

Right now, when I create a new group, one of the text fields on the form is ‘Order Quantity’ (which is a :size attribute in the Group model), and there is a line of code in the Create action for GroupsController that generates n blank orders that belong to that particular group:

  if @group.save
    @group.size.times do
      Order.create(:group_id => @group.id)
    end

This works fine, but I want Order.id to be a decimal number rather than an integer. I tried generating the following migration:

class ChangeOrderIdToDecimal < ActiveRecord::Migration
  def up
    change_column :orders, :id, :decimal, :precision => 5, :scale => 4
  end

which, at first, seemed to do what I wanted. All the existing orders changed from 4, 5, 6 to 4.0, 5.0, 6.0. What I ultimately want to do, though, is to add some code to the Create action in GroupsController like this:

  if @group.save
    order_id_decimal = 0.001
    @group.size.times do
      order_id = "#{@group.id}.#{order_id_decimal}".to_f
      Order.create(:group_id => @group.id, :id => order_id)
      order_id_decimal += 0.001
    end

The idea being, if the Group ID was 75, then the orders in that Group would have ID’s 75.001, 75.002, 75.003, etc.

For some reason, though, when I create new groups, it tries to save every order id as 0.0000 and gives me this error:

ActiveRecord::RecordNotUnique in GroupsController#create

Mysql2::Error: Duplicate entry '0.0000' for key 'PRIMARY': INSERT INTO `orders`
(`created_at`, `group_id`, `note`, `order_status`, `title`, `updated_at`, ) VALUES 
('2012-03-04 20:28:15', 20, NULL, NULL, NULL, '2012-03-04 20:28:15')

I’ve tried manually changing an Order’s ID from the rails console, and it saves without giving me errors, but it doesn’t actually save. If I type Order.find(0.0) it pulls up the one order I just changed and it has 0.0 for ID. I’m completely baffled by this, and I haven’t been able to find any help from googling it. Anyone have any ideas?

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1 Answer

  1. Editorial Team

    You’re trying to encapsulate business logic in the ID field. This is a big no-no from an application design perspective, and having an ID field that isn’t an integer is a big no-no from a database design perspective.

    I would give up on this notion that you want the IDs to be somehow correlated across tables; foreign keys and join tables should be able to accomplish the end goal (of relating groups and orders). Primary keys are not intended for business logic and should not be used for that: don’t care about the contents of your ID fields and your application will be a lot happier.

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