Rails“跟随”关系控制器问题

Question

我正在开发一个应用程序，用户的“项目”可以跟踪数据库中的“植物”对象。当我为我的应用程序中的任意数量的植物对象点击“关注”时，我在“关系”控制器（将用户的项目连接到固定的“植物”对象的植物关系）中的创建操作收到以下错误：

“当你没有预料到它时，你有一个 nil 对象！ 您可能期望有一个 Array 的实例。 评估 nil 时发生错误。[]"

我意识到这是一个大问题，是的，我几乎是个新手。所有的迁移都应该没问题。我很感激任何帮助——即使这意味着提出一种解决这个问题的全新方法。

这是我的控制器（称为“Prelationships”）的样子：

class PrelationshipsController < ApplicationController  
 def create                                                    
  @plant = Plant.find(params[:prelationship][:pfollowed_id])
  @project.follow!(@plant)
  respond_to do |format|
    format.html { redirect_to @project }
    format.js
  end
 end  
end        

还有我的“关系”模型：

class Prelationship < ActiveRecord::Base                          
 attr_accessible :pfollowed_id

   belongs_to :pfollower, :class_name => "Project"
   belongs_to :pfollowed, :class_name => "Plant"

   validates :pfollower_id, :presence => true
   validates :pfollowed_id, :presence => true
end

还有我的“项目”模型：

class Project < ActiveRecord::Base                    
attr_accessible :title, :address, :latitude, :longitude, :state      

 belongs_to :user        
 has_many :prelationships, :foreign_key => "pfollower_id",
                        :dependent => :destroy   
 has_many :pfollowing, :through => :prelationships, :source => :pfollowed  

 def pfollowing?(pfollowed)
   prelationships.find_by_pfollowed_id(pfollowed)  
 end  

 def pfollow!(pfollowed)
  prelationships.create!(:pfollowed_id => pfollowed.id)
 end
end

还有我的“植物”模型：

class Plant < ActiveRecord::Base      

 has_many :prelationships, :foreign_key => "pfollowed_id",
                        :class_name => "Prelationship"
 has_many :pfollowers, :through => :reverse_prelationships, 
                        :source => :pfollower   
end

最后，我的“_plants_form”部分视图：

<%= form_for @project.prelationships.build(:pfollowed_id =>
                                           @project_id) do |f| %>
  <%= collection_select(:prelationships, :pfollowed_id, Plant.all, :id, :name, 
  options = {:prompt => "Select your plants"}, :class => "listselect") %>  
  <div class="actions"><%= f.submit "Pfollow" %></div>
<% end %>

这是我日志中的错误：

 Started POST "/prelationships" for 127.0.0.1 at 2011-11-20 23:31:57 +0100
 Processing by PrelationshipsController#create as HTML
 Parameters: {"utf8"=>"✓",       
 "authenticity_token"=>"NKqa1f0M2yPLQDHbRLnxl3SiwBeTus/1q1hpZjD7hgY=",      
 "prelationships"=>{"pfollowed_id"=>"5"}, "commit"=>"Pfollow"}
 Completed 500 Internal Server Error in 14ms

 NoMethodError (You have a nil object when you didn't expect it!
 You might have expected an instance of Array.
 The error occurred while evaluating nil.[]):
 app/controllers/prelationships_controller.rb:4:in `create'

 Rendered /Users/mmelone12/.rvm/gems/ruby-1.9.2-p290/gems/actionpack-  
 3.0.9/lib/action_dispatch/middleware/templates/rescues/_trace.erb (1.4ms)
 Rendered /Users/mmelone12/.rvm/gems/ruby-1.9.2-p290/gems/actionpack-   
 3.0.9/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb    
 (30.6ms)
 Rendered /Users/mmelone12/.rvm/gems/ruby-1.9.2-p290/gems/actionpack- 
 3.0.9/lib/action_dispatch/middleware/templates/rescues/diagnostics.erb within 
 rescues/layout (37.1ms)

Answer 1

是的。就像 Swanand 指出的那样，您应该在 :create action 中初始化 @project 对象。喜欢@project = Project.find(params[:project_id])，如果你不使用before_filter。

如果您之前已经实例化了@project，请查看当您在Rails 控制台Plant.find(1) 中手动尝试检索@plant 对象时会发生什么