log.rb

class Log < ActiveRecord::Base
  self.table_name = "versions"
  has_one :previous_log, :class_name => "Log", 
    :order => "created_at DESC",
    :conditions => proc{ "item_type = #{self.item_type}" }
end

=> NoMethodError: undefined method `item_type' for #<Class:0x7f9f5200c5f0>

log_2.rb

class Log < ActiveRecord::Base
  self.table_name = "versions"
  has_one :previous_log, :class_name => "Log", 
    :order => "created_at DESC",
    :conditions => ['item_type = ?', '#{self.item_type}']
end

=> "SELECT  `versions`.* FROM `versions`  WHERE `versions`.`log_id` = 1 AND (item_type = '#{self.item_type}') ORDER BY created_at DESC LIMIT 1"

which also fails