问NilClass的未定义方法‘[]=’‘

EN

Properties::BuildController#update
app/controllers/properties/build_controller.rb, line 21

Started PUT "/properties/5/build/council" for 127.0.0.1 at 2013-08-18 08:52:07 +0100
Processing by Properties::BuildController#update as HTML
  Parameters: {"utf8"=>"✓","authenticity_token"=>"wBWQaxtBioqzGLkhUrstqS+cFD/xvEutXnJ0jWNtSa0=",  "council_id"=>"1", "commit"=>"Save changes", "property_id"=>"5", "id"=>"council"}
  Property Load (0.2ms)  SELECT "properties".* FROM "properties" WHERE "properties"."id" =  ? LIMIT 1  [["id", "5"]]
Completed 500 Internal Server Error in 35ms

NoMethodError - undefined method `[]=' for nil:NilClass:

<h1>Select Council</h1>


<%= form_tag  url_for(:action => 'update', :controller =>   'properties/build'), :method  => 'put' do %>

<%= select_tag :council_id, options_from_collection_for_select(Council.all, :id, :name) %>

    <%= submit_tag  %>
<% end %>

class Properties::BuildController < ApplicationController
  include Wicked::Wizard

   steps :tenant, :meter, :council, :confirmed 

  def show
    @property = Property.find(params[:property_id])
    @tenants = @property.tenants.new(params[:tenant_id])
    @meter = @property.build_meter
    @council = @property.build_council
    render_wizard
  end

  def edit
    @property = Property.find(params[:property_id])
  end


  def update
    @property = Property.find(params[:property_id])
    params[:property][:status] = step.to_s
    params[:property][:status] = 'active' if step == steps.last
    @property.update_attributes(params[:property])
    render_wizard @property
  end

end

class Council < ActiveRecord::Base
  attr_accessible :CouncilEmail, :name, :CouncilTel

  belongs_to :property
end

class Property < ActiveRecord::Base
  attr_accessible  :name, :address_attributes, :tenants_attributes, :meter_attributes, :council_attributes, :property_id, :status
  belongs_to :user 


  has_one :address, :as => :addressable
  accepts_nested_attributes_for :address, :allow_destroy => true


  has_one :council
  accepts_nested_attributes_for :council, :allow_destroy => true

  has_many :tenants, :inverse_of => :property
  accepts_nested_attributes_for :tenants, :allow_destroy => true, :reject_if     => :all_blank

  has_one :meter
  accepts_nested_attributes_for :meter, :allow_destroy => true


  validates :name,      :presence => :true
  validates :address,   :presence => :true
  validates :tenants,   :presence => true, :if => :active_or_tenants?
  validates :council,   :presence => true, :if => :active_or_council?


  def active?
    status == 'active'
  end

  def active_or_tenants?
    (status || '').include?('tenants') || active?
  end

  def active_or_council?
    (status || '').include?('council') || active?
  end
end