rails 3.2 NoMethodError undefined method `slice' for nil:NilClass

Question

i am getting this error end have no clue how to fix it. the weird thing is, it was working before. i think after i run annotate, it is broken, but not sure. the error comes from confs.controller index and own methods. it also rejects something like this: conf.machine_brand[0,1].upcase as NoMethodError [ ] bla bla this is my conf model:

# == Schema Information
#
# Table name: confs
#
#  id                 :integer          not null, primary key
#  machine_brand      :string(255)
#  machine_model      :string(255)
#  control_unit_brand :string(255)
#  control_unit_model :string(255)
#  tool_axis_x        :decimal(, )
#  tool_axis_y        :decimal(, )
#  tool_axis_z        :decimal(, )
#  rotary_axis_number :integer
#  linear_axis_number :integer
#  turning_mode       :boolean
#  milling_mode       :boolean
#  description        :text
#  xml                :text
#  user_id            :integer
#  developer_id       :integer
#  created_at         :datetime         not null
#  updated_at         :datetime         not null
#

class Conf < ActiveRecord::Base
   attr_accessible :linear_axis_number, :control_unit_brand, :control_unit_model, :description, :developer_id, :machine_brand, :machine_model, :milling_mode, :rotary_axis_number, :tool_axis_x, :tool_axis_y, :tool_axis_z, :turning_mode, :user_id, :xml 

   belongs_to :developer, :class_name => 'User', :foreign_key => 'developer_id'
   belongs_to :receiver, :class_name => 'User', :foreign_key => 'user_id'

   validates :user_id, presence: true
   validates :developer_id, presence: true
end

this is confs.controller:

class ConfsController < ApplicationController
before_filter  :signed_in_user, only:[:index, :edit, :update, :destroy]
before_filter  :developer_user, only: :destroy

def new
  @conf = Conf.new
end

def index
  @grouped = {}
  Conf.all.each do |conf|
    letter = conf.machine_brand.slice(0,1).upcase
    @grouped[letter] ||= []
    @grouped[letter] << conf
end
end

def show
@conf = Conf.find(params[:id])

respond_to do |format|
    format.html #index.html.erb
    format.json { render json: @conf }
    format.xml { render xml: @conf }  
  end
end

def own
  @grouped = {}
  Conf.where(:developer_id => current_user.id).each do |conf|
    letter = conf.machine_brand.slice(0,1).upcase
    @grouped[letter] ||= []
    @grouped[letter] << conf
  end
end

def create
@conf = Conf.new(conf_params)

  if @conf.save
    flash[:success] = "New Configuration uploaded!"
    redirect_to conf_show_path
  else
    flash[:error] = "There is a problem!"
    render 'new'
  end
end

def destroy
  @conf = Conf.find(params[:id]).destroy
  redirect_to conf_show_own_path
end

def update
  @conf.update_attributes(params[:conf])  
end

private

def signed_in_user
  unless signed_in?
    store_location
    redirect_to signin_url, notice: "Please sign in"    
  end
end

def admin_user
  redirect_to(root_path) unless current_user.admin?
end

def developer_user
  redirect_to(root_path) unless current_user.developer?
end

def conf_params
  params.require(:conf).permit(:xml, :user_id, :developer_id) if params[:conf]
end

end

and this is conf.new if you wish:

<% provide(:title, 'New Configuration')%>
<h1> Upload new configuration </h1>

<div class="row">
  <div class="span6 offset3">

     <%= form_for @conf, :html => {:multipart => true} do |f| %>

    <%= f.label :machine_brand %>
    <%= f.text_field :machine_brand %>

    <%= f.label :machine_model %>
    <%= f.text_field :machine_model %>

    <%= f.label :control_unit_brand %>
    <%= f.text_field :control_unit_brand %>

    <%= f.label :control_unit_model %>
    <%= f.text_field :control_unit_model %>

    <%= f.label :tool_axis_x %>
    <%= f.text_field :tool_axis_x %>

    <%= f.label :tool_axis_y %>
    <%= f.text_field :tool_axis_y %>

    <%= f.label :tool_axis_z %>
    <%= f.text_field :tool_axis_z %>

    <%= f.label :rotary_axis_number %>
    <%= f.text_field :rotary_axis_number %>

    <%= f.label :linear_axis_number %>
    <%= f.text_field :linear_axis_number %>

    <%= f.label :turning_mode %>
    <%= f.text_field :turning_mode %>

    <%= f.label :milling_mode %>
    <%= f.text_field :milling_mode %>

    <%= f.label :description %>
    <%= f.text_field :description %>

    <%= f.label :xml %>
    <%= f.text_field :xml %>

    <%= f.label :client %>
    <%= f.collection_select :user_id, User.where(:admin => false, :developer => false), :id, :name, options ={:prompt => "Select a client"}, :class =>"user" %>

    <%= f.label :me %>
    <%= f.collection_select :developer_id, User.where(:id => current_user.id), :id, :name, options ={:prompt => "Select me"}, :class =>"user" %>

<br />
    <%= f.submit "Upload", class: "btn btn-large btn-primary" %>
<% end %>
  </div>
</div>

Answer 1

conf.machine_brand.slice(0,1)
I think you got the error here machine_brand so simply do in your controller
letter = params[:machine_brand].to_s.slice(0,1).upcase unless params[:machine_brand].blank?
or
letter = params[:conf][:machine_brand].to_s.slice(0,1).upcase unless params[:machine_brand].blank?

Answer 2

As @Rajarshi mentioned, the error is in the following code

conf.machine_brand.slice(0,1).upcase

The error says that you're calling slice on a nil object, which means that one of your conf records has a nil machine_brand. I'm not sure how you'd want to approach this problem but this issue will be lessened if you add a validation that requires a machine_brand

class Conf < ActiveRecord::Base
  ...
  validates :machine_brand, presence: true

or you can only fetch for records where machine_brand is present

Conf.where('machine_brand IS NOT NULL').each do |conf|
  conf.machine_branch.slice(...)