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Ruby mixin for the Enum pattern

Thu, 06/03/2010 - 16:27 — Sean

Sometimes you just want to use an Enum. Unfortunately, if you're a Ruby developer, Ruby does not offer a native enum structure. Here's a simple approach using a mixin module:

module Enum
  def const_missing(key)
    @enum_hash[key]
  end
 
  def add_enum(key, value)
    @enum_hash ||= {}
    @enum_hash[key] = NameValuePair.new(value, key.to_s.downcase)
  end
 
  def each
    @enum_hash.each {|key, value| yield(key, value) }
  end
 
  def enums
    @enum_hash.keys
  end
 
  def enum_values
    @enum_hash.values
  end
 
  def get_enum_hash
    @enum_hash
  end
 
  def find_by_key(key)
    @enum_hash[key.upcase.to_sym]
  end
end

The Enum mixin depends on a NameValuePair class to hold the data:

class NameValuePair
  attr_reader :label, :value
 
  def initialize(label, value)
    @label = label
    @value = value
  end
 
  def first
    @label
  end
 
  def last
    @value
  end
end

I included first and last methods to better support the select and options_for_select helper methods in Rails. Here's how you might use it:

class FooEnum
  extend Enum
 
   self.add_enum(:APPLE, "Apple")
   self.add_enum(:PEAR, "Pear")
   self.add_enum(:ALL, "All Fruit")
end
 
FooEnum::APPLE ==> #<NameValuePair @value="apple", @label="Apple">
FooEnum::ALL.value ==> "all"
FooEnum::ALL.label ==> "All Fruit"
FooEnum.find_by_key('apple') ==> #<NameValuePair @value="apple", @label="Apple">

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