attribute_casts.rb

module AttributeCasts
  extend ActiveSupport::Concern
  module ClassMethods
    # casts method => type
    # type needs to correspond to a TypeCast class that responds to call
    def casts casts = {}
      casts.each do |meth, cast|
        caster = const_get "#{cast}_cast".camelize
        define_method "#{meth}_with_cast" do
          caster.new.call send("#{meth}_without_cast")
        end
        alias_method_chain "#{meth}", :cast
      end
    end

  end

  class BooleanCast
    def call val
      case val
      when nil, false, 0, "0", "", "false"
        false
      else
        true
      end
    end
  end

  class ArrayCast
    def call val
      case val
      when Hash
        val.values
      else
        Array.wrap(val)
      end
    end
  end

end

Attribute_defaults.rb

module AttributeDefaults
  extend ActiveSupport::Concern
  module ClassMethods
    # sets default values for accessors
    # defaults are returned whenever an accessor would return nil
    # defaults method => default_value
    def defaults defaults = {}
      defaults.each do |meth, default|
        default_proc = default.respond_to?(:to_proc) ? default.to_proc : proc { default }
        define_method "#{meth}_default" do
          default_proc.call
        end
        define_method "#{meth}_with_default" do
          res = send("#{meth}_without_default")
          !res.nil? ? res : send("#{meth}_default")
        end
        alias_method_chain meth, :default
      end
    end

  end
end

Api enhancements:

casts attribute: :method # cast by calling method on attribute
casts number: :to_i # example
casts attribute: Caster # cast by calling Caster.new
casts admin: Boolean # Boolean should be defined in namespace AttributeCasts
casts attribute: lambda { ... } # casts by calling call
casts attribute: { setter: ... } # casts on setting the attribute / default casts on getting