Can you explain what this code snippet does?

Question

I just got finished watching a railscast episode #189 and the was using a bit_mask to store a value in the users table. I am trying to implement the same thing but when I test via irb I am getting errors. First can someone explain what this is actually going. I know what the first line is doing and part of the second line

ROLES = %w[admin moderator author]

def roles=(roles)
  self.roles_mask = (roles & ROLES).map { |r| 2**ROLES.index(r) }.sum
end

def roles
  ROLES.reject { |r| ((roles_mask || 0) & 2**ROLES.index(r)).zero? }
end

In irb I get this:

irb(main):013:0> ROLES = %w[admin moderator author]
(irb):13: warning: already initialized constant ROLES
=> ["admin", "moderator", "author"]
irb(main):014:0> roles = %w[admin author]
=> ["admin", "author"]
irb(main):015:0> roles_mask = (roles & ROLES).map { |r| 2**ROLES.index(r) }.sum
NoMethodError: undefined method `sum' for [1, 4]:Array

Answer 1

He's using a number as a bit field. In binary, each bit of the number represents a flag. By adding 2^n, you can set bit n, by subtracting 2^n you can clear it (assuming it was set in the first place).

The Array#sum method might be specific to Rails, or part of Ruby 1.9. All it does is take every element in the array and add them up. You can do it this way instead.

roles_mask = (roles & ROLES).map { |r| 2**ROLES.index(r) }.inject(0){|m,n| m + n}

Answer 2

The reason that it doesn't work in irb is that sum is defined in ActiveSupport, not ruby's standard library. If you require 'active_support' first (or just use script/console), it will work in irb.

What roles= does is it turns an array of roles into a bitmask according to the order of roles in the ROLES array (item) and then saves that bitmask as role_mask. For example ["admin", "author"] will be turned into 5 (which is 0b101 in binary). Roles that aren't in ROLES will be ignored (e.g. ["admin", "bla"] is the same as just ["admin"]).

What roles does is it turns the saved bitmask back into an array of roles.