issue with associated model being duplicated on parent model update

_form.html.haml

= form_for @school_fair do |f|
  - @school_fair.event_times.each_with_index do |et, i|
    = render :partial => 'time_slot_fields', :locals => { :event_time => et, :index => i }
  = f.submit 'Save'

_time_slot_fields.html.haml

.event_time
  = fields_for "event[event_times_attributes][#{index}]", event_time do |et|
    = et.hidden_field :event_id
    %li
      = et.label :start, "Start Time and Date"
      = et.datetime_select :start
    %li
      = et.label :end_at, "End Time and Date"
      = et.datetime_select :end_at

event.rb

class Event < ActiveRecord::Base
  has_many :event_times
  accepts_nested_attributes_for :event_times, :allow_destroy => true
end

event_time.rb

class EventTime < ActiveRecord::Base
  belongs_to :event
end

events_controller.rb

class SchoolFairsController < ApplicationController
  def new
    authorize! :manage, Event
    
    @school_fair = Event.new(:event_type_id => EventType.find_by_name('school fair'))
    
    render :layout => 'admin'
  end
  
  def edit
    authorize! :manage, Event
    
    @school_fair = Event.find(params[:id])
    
    render :layout => 'admin'
  end

  def update
    authorize! :manage, Event
    
    @school_fair = Event.find(params[:id])
    
    if @school_fair.update_attributes(params[:event])
      redirect_to edit_school_fair_path(@school_fair), :notice => 'School Fair updated successfully'
    else
      render :action => 'edit'
    end
  end

end

params passed to controller

Processing by EventsController#update as HTML
  Parameters: {
  "price"=>[""], "commit"=>"Save", "authenticity_token"=>"...", "utf8"=>"✓", "id"=>"137", 
  "event"=>{
    "name"=>"fall school fair",
    "description"=>"<p>\r\n\ttest</p>\r\n",
    "event_times_attributes"=>{
"0"=>{"end_at(1i)"=>"2011", "event_id"=>"137", "total_people"=>"55", "end_at(2i)"=>"4", "end_at(3i)"=>"29", "start(1i)"=>"2011", "end_at(4i)"=>"19", "start(2i)"=>"4", "total_children"=>"0", "end_at(5i)"=>"05", "start(3i)"=>"29", "start(4i)"=>"15", "start(5i)"=>"05"}, 
"1"=>{"end_at(1i)"=>"2011", "event_id"=>"137", "total_people"=>"55", "end_at(2i)"=>"4", "end_at(3i)"=>"29", "start(1i)"=>"2011", "end_at(4i)"=>"19", "start(2i)"=>"4", "total_children"=>"0", "end_at(5i)"=>"05", "start(3i)"=>"29", "start(4i)"=>"15", "start(5i)"=>"05"}, 
"2"=>{"end_at(1i)"=>"2011", "event_id"=>"137", "total_people"=>"55", "end_at(2i)"=>"4", "end_at(3i)"=>"29", "start(1i)"=>"2011", "end_at(4i)"=>"19", "start(2i)"=>"4", "total_children"=>"0", "end_at(5i)"=>"05", "start(3i)"=>"29", "start(4i)"=>"15", "start(5i)"=>"05"}, 
"3"=>{"end_at(1i)"=>"2011", "event_id"=>"137", "total_people"=>"55", "end_at(2i)"=>"4", "end_at(3i)"=>"29", "start(1i)"=>"2011", "end_at(4i)"=>"19", "start(2i)"=>"4", "total_children"=>"0", "end_at(5i)"=>"05", "start(3i)"=>"29", "start(4i)"=>"15", "start(5i)"=>"05"}
    }
  }
}

fixed by making sure a hidden field with event_time_id is included in the partial, ala = et.hidden_field :id