當我創建的列表，我收到此錯誤

Question

未定義的方法'地圖「的零：NilClass

<% end %> 

    <div class="form-group"> 
    <%= select_tag(:location_id,options_for_select(@locations), :prompt=>"select your location")%> 
    </div> 

    <div class="form-group"> 
    <%= select_tag(:category_id,options_for_select(@categories), :prompt=>"select a category")%> 

listing_controller.rb

def new 
    @listing = Listing.new 
    @categories = Category.all.map{|c| [c.name, c.id]} 

    @locations= Location.all.map{|c| [c.name, c.id]} 
end 


def edit 
    @categories = Category.all.map{|c| [c.name, c.id]} 
    @locations= Location.all.map{|c| [c.name, c.id]} 
end 


def create 
    @listing = Listing.new(listing_params) 

    @listing.category_id = params[:category_id] 
    @listing.location_id = params[:location_id] 

    @listing.user_id = current_user.id 


    respond_to do |format| 
    if @listing.save 
     format.html { redirect_to @listing, notice: 'Listing was successfully created.' } 
     format.json { render :show, status: :created, location: @listing } 
    else 
     format.html { render :new } 
     format.json { render json: @listing.errors, status: :unprocessable_entity } 
    end 
    end 
end 

Answer 1

對於零級未定義方法映射意味着你」在沒有任何東西的情況下重新調用該方法。無類沒有這種方法。

調試代碼，找出是什麼原因造成的對象是零級

Answer 2

您可以分配值，如果其爲零或不被下面的代碼

<div class="form-group"> 
    <% unless @locations.empty? %> 
    <%= select_tag(:location_id,options_for_select(@locations), :prompt=>"select your location")%> 
    <% end %> 
</div> 

<div class="form-group"> 
    <% unless @categories.empty? %> 
    <%= select_tag(:category_id,options_for_select(@categories), :prompt=>"select a category")%> 
    <% end %> 

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