Bit Vector Preferences

Jim Morris — Tue, 07 Aug 2007 23:53:09 -0700

In my latest web project I potentially have a lot of boolean preferences, which I use for enabling or disabling various email notifications to users.

Rather than having to add a migration everytime I want to add a new preference, I thought I would use the composed_of feature in my model and compose the boolean preferences from a bitvector. That way I can simply modify my model to add new preferences rather than add new columns to the database.

I also wanted this to be easy to add new boolean preferences, so I use some Macros (I guess you could also call it Meta-Programming) to do all the repetitive code.

The result is a little class in my Person model called Preferences, one integer field in my persons database called preferences, and a composed_of :preferences in the Person model, and of course the following class in the person.rb model.

class Person < ActiveRecord::Base

  #...

  # place to store bit vector preferences
  # to add a new preference:-
  #   add symbol of preference to @@bits with bit allocation
  #   update initialize defaults if initial default is true
  class Preferences
    @@bits= {
      :comment_notifications => 1,
      :friendship_notifications => 2,
      :event_notifications => 4,
      :misc_notifications => 8 }

    # create a reader for each preference
    @@bits.each_key do |a|
      attr_reader a
    end

    # Initialize from integer or Hash
    def initialize(prefs)
      if prefs.nil?
        # set the defaults to false if not been set before
        @@bits.each do |a, v|
          instance_variable_set("@#{a}", false)  
        end
        # override default here        
        @comment_notifications= true
        @event_notifications= true
      elsif prefs.is_a?(Hash)
        # initialize from parameter Hash, and default to false if absent from hash
        @@bits.each do |a, v|
          instance_variable_set("@#{a}", false)  
        end

        prefs.each do |k,v|
          raise(ArgumentError, "Unknown preference #{k}") unless @@bits.has_key?(k.to_sym)
          instance_variable_set("@#{k}", true) if v == '1'
        end
      else
        # create from integer bit vector
        @@bits.each do |a, v|
          instance_variable_set("@#{a}", (prefs & v) != 0 ? true : false)  
        end
      end
    end

    # returns bit vector of preferences
    def preferences
      bv= 0
      @@bits.each do |a, v|
        bv |= instance_variable_get("@#{a}") ? v : 0  
      end
      return bv
    end

    # create a predicate for each preference
    @@bits.each_key do |a|
      alias_method((a.to_s + '?').to_sym, a)
    end
  end

  # access preferences as bit vector
  composed_of :preferences

  #...

end

All I need to do to add new preferences is add it to the @@bits class variable, which is a Hash of the preference name as a symbol and the bit it sets in the integer (actually the value of the bit, bit0 is 1, bit1 is 2 etc). The rest of the code is derived from the class variable.

To make things easier I also add a predicate for each preference, so I can access @person.preferences.comment_notifications? to see if any comment notifications are required for instance.

One other thing I do in the initialize method is set up defaults for the preferences. This is only really needed if it is being added as an after thought, and the column preferences is NULL in the database.

Because composed_of classes are immutable you must always create a whole new one to update them, so I also allow initialize to be called with a Hash, which can come straight from the controller. The last case of initialize is being passed the integer from the database, expanding it into the various boolean instance variables. The preferences method does the reverse and converts the boolean instance variables into the bit vector. Calls to these are all taken care of by ActiveRecord.

An example of it being called from the controller is...

@person.preferences= Person::Preferences.new(params[:preferences])

presuming you have a bunch of check boxes in your view which are passed in as part of the preferences hash.

Because I added this later I had one migration to initially add the new column...

add_column :people, :preferences, :integer

"Bit Vector Preferences" by Alex

Fri, 05 Sep 2008 00:52:19 -0700

Cool. You should mention the max amount of prefs that can be stored by this feature due to the length of an integer.

"Bit Vector Preferences" by Uri Lewin

Thu, 05 Jun 2008 13:04:21 -0700

Thanks, I was looking exactly for this functionality and it required very little effort to put it in use.

Wolfmans Howlings: Bit Vector Preferences

Bit Vector Preferences

"Bit Vector Preferences" by Alex

"Bit Vector Preferences" by Uri Lewin