[LRUG] Best way to implement an increasing number?

[Srushti Ambekallu]

Fair enough. And I can see that problem being even worse in a bulk import situation.

How about this?
	class A < ActiveRecord::Base
		has_many :bs

		def next_number
			bs.size + 1
		end
	end

	class B < ActiveRecord::Base
		belongs_to :a
		validates_presence_of :code
		before_create :set_code

		def set_code
			self.code = "#{a.name}-#{'%03d' % a.next_number}"
		end
	end

In addition to this you'll definitely need to put in a unique constraint on the table bs across code & a_id (that's what the db is for after all), and then catch any failed saves (making sure they are because of a duplicate code) and resetting the code before trying to save again. Basically, I'm trying to remove any saves of A at all.

Thanks,
Srushti
http://c42.in


On 19-Apr-2013, at 3:28 PM, Andrew Stewart wrote:

> 
> On 19 Apr 2013, at 11:55, Srushti Ambekallu wrote:
>> And possibly:
>> 	validates_uniqueness_of :code, :scope => :a_id
>> 
>> I'm guessing that would fail the save in code, rather than deadlocking in the DB, and which would then let you handle the error in code and trying another save.
> 
> Unfortunately validates_uniqueness_of doesn't really work (it's noted in the Rails docs under 'Concurrency and integrity')...
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