accepts_nested_attributes_for undefined method error

I'm trying to use accepts_nested_attributes to create a complex form. Based on the Nested Attributes documentation, examples, etc., I've set up the models like so:

User model:

require 'digest'
class User < ActiveRecord::Base
  attr_accessor :password
  attr_accessible :first_name, :last_name, :email, :password, 
                  :password_confirmation, :ducks_attributes

  has_many :ducks, :class_name => 'Duck'
  accepts_nested_attributes_for :ducks
  .
  .
  .
end

Duck model:

class Duck < ActiveRecord::Base
  belongs_to :user
  accepts_nested_attributes_for :user
end

But when I try to access the nested attributes in the console, I get

ruby-1.9.2-p290 :003 > User.first.ducks_attributes
NoMethodError: undefined method `ducks_attributes' for #<User:0x007ffc63e996e0>
    from ~/.rvm/gems/ruby-1.9.2-p290@rails3tutorial/gems/activemodel-3.0.9/lib/active_model/attribute_methods.rb:392:in `method_missing'
    from ~/.rvm/gems/ruby-1.9.2-p290@rails3tutorial/gems/activerecord-3.0.9/lib/active_record/attribute_metho开发者_JS百科ds.rb:46:in `method_missing'
    from (irb):3
    from ~/.rvm/gems/ruby-1.9.2-p290@rails3tutorial/gems/railties-3.0.9/lib/rails/commands/console.rb:44:in `start'
    from ~/.rvm/gems/ruby-1.9.2-p290@rails3tutorial/gems/railties-3.0.9/lib/rails/commands/console.rb:8:in `start'
    from ~/.rvm/gems/ruby-1.9.2-p290@rails3tutorial/gems/railties-3.0.9/lib/rails/commands.rb:23:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

What am I doing wrong? Many thanks in advance.

Only the attribute writer is defined.

class User < ActiveRecord::Base
  has_many :ducks
  accepts_nested_attributes_for :ducks
end

class Duck < ActiveRecord::Base
  belongs_to :user
end

# This works:
User.first.ducks_attributes = [ { :name => "Donald" } ]

# This is more common (attributes posted from a form):
User.create :ducks_attributes => [ { :name => "Donald" }, { :name => "Dewey" } ]