Rails Ruby on Rails

I'm not exactly sure what the cause of the problem is, so this may require some more clarification, but here's what seems to be most relevant:

I have a has_many :through and the join model has some fields that aren't foreign keys. When I build the models up and try to save I get a validation error on the non-foreign key fields from the join model.

My files look like:

Person.rb

  has_many :wedding_assignments, :dependent => :destroy
  has_many :weddings, :through=>:wedding_assignments
  accepts_nested_attributes_for :weddings
  accepts_nested_attributes_for :wedding_assignments

Wedding.rb

  has_many :wedding_assignments, :dependent => :destroy
  has_many :people, :through=>:wedding_assignments
  accepts_nested_attributes_for :people
  accepts_nested_attributes_for :wedding_assignments

WeddingAssignment.rb

  belongs_to :person
  belongs_to :wedding
  validates_presence_of :role, :person, :wedding

(role is a string)

people_controller.rb

  def new
    @person = Person.new

    1.times do
      wedding = @person.weddings.build
      1.times do
        assignment = wedding.wedding_assignments.build
        assignment.person = @person
        assignment.wedding = wedding
      end
    end
  end

  def create
    @person = Person.new(params[:person])
    @person.weddings.each do |wedding|
      wedding.wedding_assignments.each do |assignment|
        assignment.person = @person  #i don't think I should need to set person and wedding manually, but I get a validation error if I don't
        assignment.wedding = wedding
      end
    end
 end

the params that come back look like:

{"first_name"=>"", "last_name"=>"", "weddings_attributes"=>{"0"=>{"wedding_assignments_attributes"=>{"0"=>{"role"=>"Bride's Maid", "budget"=>""}}, "date"=>"", "ceremony_notes"=>""}}}

And the exact error is:

ActiveRecord::RecordInvalid in PeopleController#create
Validation failed: Role can't be blank

Which is clearly not correct, since you can see it in params[]

The expected behavior of course is for the save to succeed. This can be duplicated in the console. Inspecting the value of person.weddings.first.wedding_assignments.first.role gives "Bride's Maid", so I know it's there.

This is rails 3.0.0 on a mac

I'm not sure what else I can check to debug this. I would really appreciate some advice.

Thank you so much for the beautiful thing that is rails 3!