Seeding a Rails Database with Pre-set IDs

Question

I have an XML file containing seed data that I'm trying to load into my production database.

The problem being that I have associations between different data types / nodes in the XML file, and so I need the specified IDs to be the same when loaded into MySQL from Rails.

This works fine with SQLite during development, I just use a line like the one below after iterating through each node in the XML file:

CardSet.connection.execute("UPDATE card_sets SET id = #{xml_set.attributes['id']} WHERE id = #{set.id}")

My question is: how can I force pre-set IDs when seeding the database, and then be able to turn auto_increment back on for Rails to function as normal?

I've attempted this with these two lines before creating the database entry:

CardSet.connection.execute("ALTER TABLE card_sets CHANGE id id INT(11) DEFAULT NULL")
CardSet.connection.execute("ALTER TABLE card_sets DROP PRIMARY KEY")

and this after:

CardSet.connection.execute("ALTER TABLE card_sets CHANGE id id INT(11) DEFAULT NULL auto_increment PRIMARY KEY")

which MySQL fires back to me as:

Mysql::Error: ALTER TABLE causes auto_increment resequencing, resulting in duplicate entry '18' for key 'PRIMARY': ALTER TABLE card_sets CHANGE id id INT(11) DEFAULT NULL auto_increment PRIMARY KEY

My schema:

create_table "card_sets", :force => true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "card_sets_cards", :id => false, :force => true do |t|
  t.integer "card_set_id"
  t.integer "card_id"
end

create_table "cards", :force => true do |t|
  t.text     "question",   :default => ""
  t.text     "answer",     :default => ""
  t.datetime "created_at"
  t.datetime "updated_at"
end

And models:

class CardSet < ActiveRecord::Base
  has_and_belongs_to_many :cards, :uniq => true
end

class Card < ActiveRecord::Base
    has_and_belongs_to_many :card_set, :uniq => true
end

Any ideas would be appreciated.

Answer 1

If you create your objects in block form, it'll work.

p = Post.new do |post|
  post.id = 1000
end

p.save

Answer 2

My way around the MySQL error was to create the model instance normally (CardSet.create), and then force an update to the ID, and then reload the instance:

c = CardSet.create(...blah....)
CardSet.connection.execute("UPDATE card_sets SET id = #{real_card_set_id} WHERE id = #{c.id}")
c = CardSet.find(real_card_set_id)
c.save!