incorrect nested model save error

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!