Question

Edit:

Is it possible to create a unique auto increment field that will be incremented on creates and updates in SQL using Rails (similar to an id field but incremented and re-assigned after an update)? For example:

Create Record A (Value: 1)
Create Record B (Value: 2)
Update Record A (Value: 3)
Update Record B (Value: 4)

I’m trying to setup pull synchronization and need a way to grab all records that have been created or updated since a previous synchronization.

I initially used the ‘created_at’ and ‘updated_at’ fields, but found them to be difficult to work with and somewhat inaccurate for partial synchronizations.

Edit:

I’m using Postgresql and Sqlite as my databases, so hopefully a solution exists that will work for both systems.

Edit:

To clarify, I want to pass a single integer to my server from the client (the largest ‘sync’ integer) and get back all the records created or updated after that record was created or updated.

Answer 1

Ended up adding a sequence integer field to my model and setup the following migration:

class CreateSequence < ActiveRecord::Migration
  def self.up
    begin
      execute "CREATE SEQUENCE sequence"
    rescue
    end
  end

  def self.down
    begin
      execute "DROP SEQUENCE sequence"
    rescue
    end
  end
end

Then, in my model I added:

before_save do
  self.sequence = self.class.sequence
end

def self.sequence
  s ||= self.connection.select_value("SELECT nextval('sequence') ") rescue nil
  s ||= self.connection.select_value("SELECT strftime('%s','now')") rescue nil
  return 
end

Note: For Sqlite sequences are not supported so instead a selection of an ‘epoch’ form the database is required. However, this has the negative side effect of causing the sequence to be non-unique for rapid creation. However, in my case this was not an issue.