Nested Model Mass Assignment Error

Hi,

I have a many-to-many relationship via a join table. When I attempt to
set the values (i.e. the relationship) in the join table via the Rails
console I get the following error, does anyone know what could be wrong?
Thanks.

Note: I have set attr_accessible -- see Grade model below.

ActiveModel::MassAssignmentSecurity::Error: Can't mass-assign protected
attributes: id, grade_salary_attributes_attributes

In the Rails console I execute:

grade = Grade.find(:first)

params = {:grade => { :id => 1, :grade_salary_attributes_attributes => {
:id => 1, :grade_id => 1, :salary_attribute_id => 2 }}}

grade.update_attributes(params[:grade])

Below are the 3 models:

**Grade Model**

class Grade < ActiveRecord::Base
  has_many :grade_salary_attributes
  has_many :salary_attributes, :through => :grade_salary_attributes

  accepts_nested_attributes_for :grade_salary_attributes, :reject_if =>
lambda {|attributed| attributed['grade_id'].nil? ||
attributed['salary_attribute_id'].nil?}

  attr_accessible :description, :title,
:grade_salary_attributes_attributes
end

**Salary Attribute Model**

class SalaryAttribute < ActiveRecord::Base
  has_many :grade_salary_attributes
  has_many :grades, :through => :grade_salary_attributes

  attr_accessible :description, :title
end

**Grade Salary Attribute (Join) Model**

class GradeSalaryAttribute < ActiveRecord::Base
  belongs_to :grade
  belongs_to :salary_attribute

  attr_accessible :grade_id, :salary_attribute_id
end