def generate_id
  "#{environment_id}_#{app_id}_#{server_id}"
end

pry(main)> AppServer.first.generate_id
=> "2_3_1"

pry(main)> AppServer.first.app_server_id = AppServer.first.generate_id
=> "2_3_1"
pry(main)> AppServer.first.app_server_id
=> nil

pry(main)> AppServer.first.update_attribute(:app_server_id, AppServer.first.generate_id)
NoMethodError: undefined method `eq' for nil:NilClass
from C:/Ruby192/lib/ruby/gems/1.9.1/gems/activesupport-3.0.9/lib/active_support/whiny_nil.rb:48:in `method_missing'

pry(main)> apps=AppServer.first
=> #<AppServer app_id: 3, server_id: 1, environment_id: 2, app_server_id: nil>
pry(main)> apps.app_server_id = apps.generate_id
=> "2_3_1"
pry(main)> apps.save
NoMethodError: undefined method `eq' for nil:NilClass
from C:/Ruby192/lib/ruby/gems/1.9.1/gems/activesupport-3.0.9/lib/active_support/whiny_nil.rb:48:in `method_missing'

pry(main)> AppServer.first.app_server_id = AppServer.first.generate_id
=> "2_3_1"
pry(main)> AppServer.first.app_server_id
=> nil

undefined method `eq' for nil:NilClass

def self.up
  execute "ALTER TABLE 'app_servers'
              ADD COLUMN 'id' INT(11) AUTO_INCREMENT NOT NULL FIRST,
              ADD PRIMARY KEY('id')"
end