Comment functionality in Rails. undefined method `first_name' for nil:NilClass - ruby-on-rails

Question

One thing I could never do properly is implement a comment feature. I'm not leaving my computer until I learn to do it.
The error is thrown on this line:
<strong><%= comment.user.first_name %></strong>
Apparently user is nil; but why? And what do I have to do to get this to work?
A comment should belong to a guide and a user. Users and guides both have many comments.
I started with
rails g scaffold comment body:text guide:references user:references
and then migrated the database. I completed the model associations as well.
Here is my guides controller show action:
def show
#guide = Guide.find(params[:id])
#comment = #guide.comments.build
end
Here is the part of the Guide show view that deals with comments:
<h3>Comments</h3>
<% #guide.comments.each do |comment| %>
<div>
<strong><%= comment.user.first_name %></strong>
<br />
<p><%= comment.body %></p>
</div>
<% end %>
<%= render 'comments/form' %>
Here is the comment form partial:
<%= simple_form_for(#comment) do |f| %>
<%= f.error_notification %>
<div class="form-inputs">
<%= f.input :body %>
<%= f.association :user %>
<%= f.association :guide %>
</div>
<div class="form-actions">
<%= f.button :submit %>
</div>
<% end %>
User.rb
class User < ActiveRecord::Base
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable, #:recoverable,
:rememberable, :trackable, :validatable
validates :first_name, presence: true
validates :email, presence: true
validates :email, uniqueness: true
validates :runescape_username, presence: true
has_many :guides
has_many :comments
acts_as_voter
def user_score
self.guides.inject(0) { |sum, guide| sum += guide.score }
end
end
Comment.rb
class Comment < ActiveRecord::Base
belongs_to :user
belongs_to :guide
end
Comments controller create action:
def create
#comment = Comment.new(comment_params)
respond_to do |format|
if #comment.save
format.html { redirect_to #comment, 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

Answer 1

Replace the line
#comment = Comment.new(comment_params)
with
#comment = current_user.comments.build(comment_params)
in your Comments#create action.
You get this error because you don't assign current_user to created Comment. That's why comment.user returns nil.
As stated by AndreDurao, you can also validate user_id presence in Comment model, like this:
class Comment
validates_presence_of :user
# ...
end

Answer 2

for getting rid of that error try this <%= comment.user.try(:first_name) %>