acts_as_tree test fails : duplicate entry # for key #

Hi,
I've set up a model as follows

class EquipmentPrice < ActiveRecord::Base

  acts_as_tree :order => "id"

  ...
end

which has the following db migration
class CreateEquipmentPrices < ActiveRecord::Migration
  def self.up
    create_table :equipment_prices do |t|
      t.integer :id
      t.string :manufacturer
      t.string :model
      t.string :equipment_type
      t.integer :capacity
      t.integer :string_size
      t.string :cost_structure
      t.string :currency
      t.decimal :price_each
      t.decimal :price_bulk
      t.integer :bulk_quantity
      t.integer :parent_id

      t.timestamps
    end
  end

  def self.down
    drop_table :equipment_prices
  end
end

But when I try and run the default test, I get an error message
stating
ActiveRecord::StatementInvalid: Mysql::Error: Duplicate entry '1' for
key 1: INSERT INTO `equipment_prices` (`id`, `manufacturer`, `model`,
`equipment_type`, `capacity`, `string_size`, `cost_structure`,
`currency`, `price_each`, `price_bulk`, `bulk_quantity`, `parent_id`,
`created_at`, `updated_at`) VALUES (1, NULL, NULL, 'PV_subsystem',
NULL, NULL, NULL, 'USD', 0.0, 0.0, 1, 0, '2010-03-11 12:14:05',
'2010-03-11 12:14:05')

I have read in an earlier post that this is because I have parent_id
set as the primary key for the table. But when I look at the mysql
table definition I see

resolved - it would appear that MySQL treats an int of 0 = NULL. , so
my first fixture (the root of the tree) which had an id of 0 in the
fixtures file, was being taken as NULL (stated as not allowed in the
table definition), so set to id=1 by MySQL, then causing the 2nd
fixture (id=1 in the fixtures file) to appear as a duplicate.