Rails5からはもうdefault_value_forは必要ないのかもしれない ref: http://qiita.com/inuscript/items/e98caa8469ed7143404a
class SomeModel < ActiveRecord::Base
attribute :some_flag, :boolean, default: -> { false }
end
class SomeModel < ActiveRecord::Base
default_value_for :is_public, false
end