how to create dropdown from a hash in rails 3

In rails 3, how to create a Dropdown from hash

I have following code in my User class

class User
  ... other codes
  key :gender, Integer # i use mongo db

  class << self
    def genders() # not sure how to define static value, so do in
this way
      genders = {
        '1' => 'Male',
        '2' => 'Female',
        '3' => 'Secret'
      }
    end
  end

end

In the user form, i am trying to create a gender dropdown list
<%= f.collection_select nil, :gender, User.genders, :key, :value %>

but it complain
undefined method `merge' for :value:Symbol

So what is the proper way to create the dropdown?

Thanks