Returns undefined method 'each' for nil:NilClass?

Question

Here is my code in the view: show.html.erb

<ul>
<% @bullets.each do |r| %>
    <li><%= r.content %></li>
<% end %>
</ul>

Here is my code in the controller: users_controller.rb

if cookies[:bullets].nil?
  @bullets = Bullet.all.shuffle.first(4)
  cookies[:bullets] = @bullets.collect(&:id)
else
 @bullets = []
 cookies[:bullets].each do |id|
   @bullets << Bullet.find(id)
 end
end

This returns undefined method 'each' for nil:NilClass on

<% @bullets.each do |r| %>

I would like to know why it does this, and how can I fix it to post four random fixed bullet contents from a database (sqlite3) table named "bullets" (column is content).

EDIT: This is the ENTIRE controller:

class StudentsController < ApplicationController
    #GET /
    def index
      @students = Student.all 

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

    #GET /new
    def new 
      @student = Student.new
    end 

    #POST
    def create
      @student = Student.new(params[:student])
          if @student.save
        render :file => 'app/views/success'
      else 
        render :file => 'app/views/students/fail'
      end  
        end

    #GET /students/{:id}
    def show 
       @student = Student.find_by_url(params[:id])

           respond_to do |format|
         format.html # show.html.erb
         format.json { render json: @student } 
           end  
    end

    #BULLETS Randomizing /students/new.html.erb
    if cookies[:bullets].nil?
      @bullets = Bullet.all.shuffle.first(4)
      cookies[:bullets] = @bullets.collect(&:id)
    else
     @bullets = []
     cookies[:bullets].each do |id|
       @bullets << Bullet.find(id)
     end
    end

    #GET /students/1/edit
    def edit 
        @student = Student.find_by_url(params[:id])
    end 

    def update
      @student = Student.find_by_url(params[:id])
      respond_to do |format|
        if @student.update_attributes(params[:student])
          format.html { redirect_to @student, notice: 'Student was successfully updated.'}  
        else 
          format.html { render action: "edit" } 
          format.json { render json: @student.errors, status: :unprocessable_entity } 
        end 
        end 
    end 

    #DELETE
    def destroy 
    @student = Student.find_by_url(params[:id])
    @student.destroy

        respond_to do |format|
         format.html { redirect_to students_url }
         format.json { head :no_content }
        end
    end
end 

EDIT #2: Like so?

#GET /students/{:id}
    def show 
       @student = Student.find_by_url(params[:id])

        #BULLETS Randomizing /students/show.html.erb
    if cookies[:bullets].nil?
      @bullets = Bullet.all.shuffle.first(4)
      cookies[:bullets] = @bullets.collect(&:id)
    else
     @bullets = []
     cookies[:bullets].each do |id|
       @bullets << Bullet.find(id)
     end
    end

           respond_to do |format|
         format.html # show.html.erb
         format.json { render json: @student } 
           end  
    end

Answer 1

Looks like it should be:

#GET /students/{:id}
def show 

   @student = Student.find_by_url(params[:id])

   #BULLETS Randomizing /students/new.html.erb
   if cookies[:bullets].nil?
     @bullets = Bullet.all.shuffle.first(4)
     cookies[:bullets] = @bullets.collect(&:id)
   else
     # simpler to use an 'in list' for only 4 id's
     Bullet.where("id in (?)", cookies[:bullets])
   end

   respond_to do |format|
     format.html # show.html.erb
     format.json { render json: @student } 
   end  
end

Notice I converted your loop over the cookies array into a single statement with an 'in list' -- that should simplify the sql generated for the lookups.

Although, it's arguable that this code should be pushed into the model:

class Bullet < ActiveRecord::Base

  NUM_USER_BULLETS = 4
  # fetch a random set of 
  def self.user_bullets
    Bullet.all.shuffle.first(NUM_USER_BULLETS)
  end
end

Or something similar. Then your controller is simpler:

#GET /students/{:id}
def show 

   @student = Student.find_by_url(params[:id])

   #BULLETS Randomizing /students/new.html.erb
   if cookies[:bullets].nil?
     @bullets = Bullet.user_bullets
     cookies[:bullets] = @bullets.collect(&:id)
   else
     # simpler to use an 'in list' for only 4 id's
     Bullet.where("id in (?)", cookies[:bullets])
   end

   respond_to do |format|
     format.html # show.html.erb
     format.json { render json: @student } 
   end  
end

With the code migrated into your model, your controller is simpler.

Answer 2

class StudentsController < ApplicationController
   before_filter :get_bullets, :only => [:show]


  def show

  end

  ... 


  protected

  def get_bullets
  #BULLETS Randomizing /students/new.html.erb
    if cookies[:bullets].nil?
      @bullets = Bullet.all.shuffle.first(4)
      cookies[:bullets] = @bullets.collect(&:id)
    else
     @bullets = []
     cookies[:bullets].each do |id|
       @bullets << Bullet.find(id)
     end
    end

  end



end

Answer 3

I would suggest refactoring your "random bullets" code into a before_filter for the show action and performing the randomization at the database, which will be faster than loading the entire table and performing the randomization in Ruby:

class UsersController < ApplicationController
  before_filter :assign_bullets, :only => :show

  # ...

private

  def assign_bullets
    if cookies[:bullets]
      @bullets = cookies[:bullets].map { |id| Bullet.find(id) }
    else
      @bullets = Bullet.order('RANDOM()').limit(4)
      cookies[:bullets] = @bullets.map(&:id)
    end
  end
end