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?