How to log something in Rails in an independent log file?

In rails I want to log some information in a different log file and not the standard development.log or production.log. I want to do this logging from a model class.

You can create a Logger object yourself from inside any model. Just pass the file name to the constructor and use the object like the usual Rails logger:

class User < ActiveRecord::Base
  def my_logger
    @@my_logger ||= Logger.new("#{Rails.root}/log/my.log")
  end

  def before_save
    my_logger.info("Creating user with name #{self.name}")
  end
end

Here I used a class attribute to memoize the logger. This way it won't be created for every single User object that gets created, but you aren't required to do that. Remember also that you can inject the my_logger method directly into the ActiveRecord::Base class (or into some superclass of your own if you don't like to monkey patch too much) to share the code between your app's models.

Update

I made a gem based on the solution below, called multi_logger. Just do this in the initializer:

MultiLogger.add_logger('post')

and call

Rails.logger.post.error('hi')
# or call logger.post.error('hi') if it is accessible.

and you are done.

If you want to code it yourself, see below:

A more complete solution would be to place the following in your lib/ or config/initializers/ directory.

The benefit is that you can setup formatter to prefix timestamps or severity to the logs automatically. This is accessible from anywhere in Rails, and looks neater by using the singleton pattern.

# Custom Post logger
require 'singleton'
class PostLogger < Logger
  include Singleton

  def initialize
    super(Rails.root.join('log/post_error.log'))
    self.formatter = formatter()
    self
  end

  # Optional, but good for prefixing timestamps automatically
  def formatter
    Proc.new{|severity, time, progname, msg|
      formatted_severity = sprintf("%-5s",severity.to_s)
      formatted_time = time.strftime("%Y-%m-%d %H:%M:%S")
      "[#{formatted_severity} #{formatted_time} #{$$}] #{msg.to_s.strip}\n"
    }
  end

  class << self
    delegate :error, :debug, :fatal, :info, :warn, :add, :log, :to => :instance
  end
end

PostLogger.error('hi')
# [ERROR 2012-09-12 10:40:15] hi