ruby-on-rails – Rails：更新嵌套属性 – nil的未定义方法`to_s

模型：

class Match < ActiveRecord::Base  
  has_and_belongs_to_many :teams
  has_many :match_teams
  has_many :teams,:through => :match_teams
  accepts_nested_attributes_for :match_teams,:allow_destroy => true
end

控制器：

def new
    @match = Match.new
    @match_teams = 2.times do
      @match.match_teams.build
    end

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @match }
    end
  end

  def update
    @match = Match.find(params[:id])

    respond_to do |format|
      if @match.update_attributes(params[:match])
        format.html { redirect_to @match,notice: 'Match was successfully updated.' }
        format.json { head :ok }
      else
        format.html { render action: "edit" }
        format.json { render json: @match.errors,status: :unprocessable_entity }
      end
    end
  end

嵌套模型：

class MatchTeam < ActiveRecord::Base
  belongs_to :match
  belongs_to :team
end

协会：

class Team < ActiveRecord::Base
  has_and_belongs_to_many :matches
end

视图：

<%= form_for(@match) do |f| %>

  <%= f.fields_for :match_teams,@match_teams do |builder| %>
    <%= builder.collection_select :team_id,Team.all,:id,:name,:include_blank => true %>
  <% end %>

  <% unless @match.new_record? %>
    <div class="field">
      <%= f.label :winning_team_id %><br />
      <%= f.collection_select :winning_team_id,@match.teams,:representation %>
    </div>
  <% end %>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

PARAMS：

Processing by MatchesController#update as HTML
Parameters: {"utf8"=>"?￡?","authenticity_token"=>"QIJChzkYOPZ1hxbzTZS8H3AXc7i
BzkKv3Z5daRmlOsQ=","match"=>{"match_teams_attributes"=>{"0"=>{"team_id"=>"1","
id"=>""},"1"=>{"team_id"=>"3","id"=>""}},"winning_team_id"=>"3"},"commit"=>"
Update Match","id"=>"2"}

创建与2个团队的新匹配工作正常,编辑视图也显示正确的值,但更新操作给我这个错误.

undefined method `to_sym' for nil:NilClass
app/controllers/matches_controller.rb:65:in `block in update'

line 65: if @match.update_attributes(params[:match])