Rails undefined方法`[]'为nil：NilClass

Question

The following is my current code. I removed parts not needed to understand the problem.

Setting Model

class Setting < ActiveRecord::Base
  attr_accessor :program_id, :session_types

  def initialize
    @session_types = Array.new
    @program_id = ''
  end
end

Setting Controller

  def new
    @setting = Setting.new
    puts @setting.session_types.instance_of? Array
  end

Setting View

<%= form_for(@setting) do |f| %> #ERROR LINE
    ...
<% end %>

I get this error:

ActionView::Template::Error (undefined method `[]' for nil:NilClass):
    3: </h1>
    4: <p>We need to get your booking page setup. Please select the Program you wish to put up for booking.</p>
    5: 
    6: <%= form_for(@setting) do |f| %>
    7:     <% @programs.each do |program| %>
    8:         <%= f.radio_button(:program_id, program.program_id)%>
    9:         <%= label_tag(:program_name, program.name) %>
  app/views/settings/new.html.erb:6:in `_app_views_settings_new_html_erb__94314060479739781_70294544827460'

@setting.session_types = Empty Array when debugging.

So I tried replacing Setting Model with a simple Testing Model without the array attribute as follows:

class Testing < ActiveRecord::Base
  attr_accessor :program_id
end

to see if it is the Array attribute session_types of Setting that is causing the error. It seems so becuase with Testing , I get an error in the line AFTER the form_for line, which should be correct. (the line is not needed to undestand the problem)

I think I am not understanding the initialization of an Array attribute of a model properly. Any help to understand why the error is thrown will be appreciated! Thanks in advance! =D

Answer 1

Try this

def initialize
  super
  @session_types = Array.new
  @program_id = ''
end

You were very scarce with pasting your errors, but that's the first thing that comes to mind. Please post the entire error if this doesn't help.

The reason for that is your Setting class inherits from ActiveRecord::Base , calling super first calls parent's initialize method, this way you add functionality to the method (extend it). Without super you override it, meaning you replace everything it does by default (like for example connecting to the database, validating) by your code.

Answer 2

Do not override initialize method on ActiveRecord::Base . Use after_initialize callback instead. Reference possibly with your problem http://blog.dalethatcher.com/2008/03/rails-dont-override-initialize-on.html