Implementation ideas for a unique identifier

I have an Item model that uses acts_as_nested_set (via better_nested_set plugin) to maintain tree structures of Items. There can potentially be thousands of "root" Items. A requirement is that each "root" Item have a unique identifier in the form 'YYMMDD-nnnnnn' where nnnnnn is a sequential counter (e.g. 071106-000024). There can be no gaps in this sequential counter.

For example, a "root" Item with 57 children is added to the database. The "root" has a unique identifier of '071106-000001' because it is the first "root" item (Item#id = 1). The next "root" item (Item#id = 58) must have a unique identifier of '071106-000002' and NOT '071106-000058'.

How would you implement this "sequential counter"?

The rows have their own regular integer keys, and that's an additional column that is NULL for non-root nodes?

-- fxn

Correct.

Sometimes writing it out in the form of a question gets the creative juices flowing.What if I added an attribute to the items table named "sequence". Then I could use an after_create callback and two private methods such as:

  def after_create     if self.root?       update_attribute(:sequence, generate_sequence)       update_attribute(:identifier, generate_identifier)     end   end

  private

  def generate_sequence     Item.maximum(:sequence, :conditions => ['parent_id != ?', 'NULL']) + 1   end

  def generate_identifier     "#{self.created_on.strftime('%y%m%d')}- #{self.sequence.to_s.rjust(6, '0')}"   end

Here is my working solution:

class AddSequenceToItems < ActiveRecord::Migration   def self.up     add_column :items, :sequence, :integer   end

  def self.down     remove_column :items, :sequence   end end

class Item < ActiveRecord::Base   def after_create     # This item doesn't already have a root, so this is a new root item     update_attribute(:root_id, self.id) if self.root_id.nil?

    if root?       update_attribute(:sequence, generate_sequence)       update_attribute(:identifier, generate_identifier)     end   end

  def sequence(padded = false)     padded ? padded_sequence : self[:sequence]   end

  private

  def self.maximum_sequence     Item.maximum(:sequence) || 0   end

  def generate_sequence     self.class.maximum_sequence + 1   end

  def generate_identifier     "#{self.created_on.strftime('%y%m%d')}-#{padded_sequence}"   end

  def padded_sequence(length = 6, str = '0')     self[:sequence].to_s.rjust(length, str)   end end

I have an Item model that uses acts_as_nested_set (via better_nested_set plugin) to maintain tree structures of Items. There can potentially be thousands of "root" Items. A requirement is that each "root" Item have a unique identifier in the form 'YYMMDD-nnnnnn' where nnnnnn is a sequential counter (e.g. 071106-000024). There can be no gaps in this sequential counter.

For example, a "root" Item with 57 children is added to the database. The "root" has a unique identifier of '071106-000001' because it is the first "root" item (Item#id = 1). The next "root" item (Item#id = 58) must have a unique identifier of '071106-000002' and NOT '071106-000058'.

How would you implement this "sequential counter"?

Have a table with two columns. The first is the date. The second is an integer.

Write a routine that gets you the next highest value for the given date and then updates the table with that new value for next time. And put a gigantic lock around the entire thing so that you don't have any race conditions.

If it were me, I'd spend some time trying to remove that restriction, but I'm guessing you're stuck with it.

What happens in this situation:

- Lock. - Get the next NNNN value. - Update the database for next time. - Unlock. - Try and use it, but it freaks out, entire Rails app crashes.

- Restart app.

At this point you have a hole in your sequence... how do you even know if you have a hole? What do you do?