undefined method `first_name' for nil:NilClass Error is in show.html.erb on line 4

_form.html.erb

<%= simple_form_for(@comment) do |f| %>
  <%= f.error_notification %>

  <div class="form-inputs">
    <%= f.input :body %>
    <%= f.input :guide_id %>
    <%= f.input :user_id %>
  </div>

    <%= f.hidden_field :guide_id %>
    <%= f.hidden_field :user_id %>


  <div class="form-actions">
    <%= f.button :submit %>
  </div>

<% end %>

comment.rb

class Comment < ActiveRecord::Base
  belongs_to :user
  belongs_to :guide
end

comments_controller.rb

class CommentsController < ApplicationController
  before_action :set_comment, only: [:show, :edit, :update, :destroy]

  # GET /comments
  # GET /comments.json
  def index
    @comments = Comment.all
  end

  # GET /comments/1
  # GET /comments/1.json
  def show
  end

  # GET /comments/new
  def new
    @comment = Comment.new
  end

  # GET /comments/1/edit
  def edit
  end

  # POST /comments
  # POST /comments.json
  def create
      @comment = Comment.new(comment_params.merge({:user_id => current_user.id}))

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

    end

  # PATCH/PUT /comments/1
  # PATCH/PUT /comments/1.json
  def update
    respond_to do |format|
      if @comment.update(comment_params)
        format.html { redirect_to @comment, notice: 'Comment was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @comment.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /comments/1
  # DELETE /comments/1.json
  def destroy
    @comment.destroy
    respond_to do |format|
      format.html { redirect_to comments_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_comment
      @comment = Comment.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def comment_params
      params.require(:comment).permit(:body, :guide_id, :user_id)
    end
end

comments_migration.rb

class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.text :body
      t.integer :guide_id
      t.integer :user_id

      t.timestamps
    end
    add_foreign_key :comments, :guides
  end
end

guides_controller.rb

class GuidesController < ApplicationController
  before_action :set_guide, only: [:show, :edit, :update, :destroy]
  before_action :authenticate_user!, except: [:index, :show]

  # GET /guides
  # GET /guides.json
  def index
    if params[:tag]
      @guides = Guide.tagged_with(params[:tag])
    else
      @guides = Guide.all
    end

  end

  # GET /guides/1
  # GET /guides/1.json
  def show
    @guide = Guide.find(params[:id])
    @comments = guide.comments
    @comment = comments.build
  end

  # GET /guides/new
  def new
    @guide = current_user.guides.build(guide_params)
  end

  # GET /guides/1/edit
  def edit
  end

  # POST /guides
  # POST /guides.json
  def create
    @guide = current_user.guides.build(guide_params)

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

  # PATCH/PUT /guides/1
  # PATCH/PUT /guides/1.json
  def update
    respond_to do |format|
      if @guide.update(guide_params)
        format.html { redirect_to @guide, notice: 'Guide was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @guide.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /guides/1
  # DELETE /guides/1.json
  def destroy
    @guide.destroy
    respond_to do |format|
      format.html { redirect_to guides_url }
      format.json { head :no_content }
    end
  end

  def upvote
    @guide = Guide.find(params[:id])
    @guide.liked_by current_user
    redirect_to :back
  end

  def downvote
    @guide = Guide.find(params[:id])
    @guide.downvote_from current_user
    redirect_to :back
  end




  private
    # Use callbacks to share common setup or constraints between actions.
    def set_guide
      @guide = Guide.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def guide_params
      params.require(:guide).permit(:title, :author, :description, :link, :tag_list) if params[:guide]
    end
end

schema.rb

# encoding: UTF-8
# This file is auto-generated from the current state of the database. Instead
# of editing this file, please use the migrations feature of Active Record to
# incrementally modify your database, and then regenerate this schema definition.
#
# Note that this schema.rb definition is the authoritative source for your
# database schema. If you need to create the application database on another
# system, you should be using db:schema:load, not running all the migrations
# from scratch. The latter is a flawed and unsustainable approach (the more migrations
# you'll amass, the slower it'll run and the greater likelihood for issues).
#
# It's strongly recommended that you check this file into your version control system.

ActiveRecord::Schema.define(version: 20140103130329) do

  create_table "comments", force: true do |t|
    t.text     "body"
    t.integer  "guide_id"
    t.integer  "user_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "favorite_guides", force: true do |t|
    t.integer  "guide_id"
    t.integer  "user_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "guides", force: true do |t|
    t.string   "title"
    t.string   "author"
    t.text     "description"
    t.string   "link"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.integer  "user_id"
  end

  add_index "guides", ["user_id"], name: "index_guides_on_user_id"

  create_table "taggings", force: true do |t|
    t.integer  "tag_id"
    t.integer  "taggable_id"
    t.string   "taggable_type"
    t.integer  "tagger_id"
    t.string   "tagger_type"
    t.string   "context",       limit: 128
    t.datetime "created_at"
  end

  add_index "taggings", ["tag_id"], name: "index_taggings_on_tag_id"
  add_index "taggings", ["taggable_id", "taggable_type", "context"], name: "index_taggings_on_taggable_id_and_taggable_type_and_context"

  create_table "tags", force: true do |t|
    t.string "name"
  end

  create_table "users", force: true do |t|
    t.string   "first_name"
    t.string   "runescape_username"
    t.string   "email",                  default: "", null: false
    t.string   "encrypted_password",     default: "", null: false
    t.string   "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer  "sign_in_count",          default: 0,  null: false
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.string   "current_sign_in_ip"
    t.string   "last_sign_in_ip"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  add_index "users", ["email"], name: "index_users_on_email", unique: true
  add_index "users", ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true

  create_table "votes", force: true do |t|
    t.integer  "votable_id"
    t.string   "votable_type"
    t.integer  "voter_id"
    t.string   "voter_type"
    t.boolean  "vote_flag"
    t.string   "vote_scope"
    t.integer  "vote_weight"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  add_index "votes", ["votable_id", "votable_type", "vote_scope"], name: "index_votes_on_votable_id_and_votable_type_and_vote_scope"
  add_index "votes", ["voter_id", "voter_type", "vote_scope"], name: "index_votes_on_voter_id_and_voter_type_and_vote_scope"

end

show.html.erb

<h3>Comments</h3>
  <% @guide.comments.each do |comment| %>
    <div>
      <strong><%= comment.user.first_name %></strong> :
      <p><%= comment.body %></p>
    </div>
  <% end %>
  <%= render 'comments/form' %>

user.rb

class User < ActiveRecord::Base
  has_many :guides
  has_many :comments
end