undefined method `first' for #<Article:0x5b0a4d8> - ruby-on-rails

Question

I am creating a blog with tags. Using Mongodb.
I got this error when submitting a new article with tags. And new artcile is not created. If I do not put any tags in the form, the article is created.
NoMethodError in ArticlesController#create
undefined method `first' for #
Rails.root: c:/Sites/blogK
Application Trace | Framework Trace | Full Trace
app/models/article.rb:18:in all_tags='
app/controllers/articles_controller.rb:22:innew'
app/controllers/articles_controller.rb:22:in `create'
article.rb
class Article
include Mongoid::Document
has_many :comments, dependent: :destroy
field :title, type: String
field :text, type: String
embeds_many :taggings
embeds_many :tags
def all_tags=(tags_string)
tag_names = tags_string.split(",").collect{|s| s.strip.downcase}.uniq
new_or_found_tags = tag_names.collect { |name| Tag.find_or_create_by(name: name) }
self.tags = new_or_found_tags
end
def all_tags
self.tags.collect do |tag|
tag.name
end.join(", ")
end
validates :title, presence: true,
length: { minimum: 5 }
end
tag.rb
class Tag
include Mongoid::Document
field :name, type: String
embeds_many :taggings
embeds_many :articles
end
tagging.rb
class Tagging
include Mongoid::Document
embedded_in :article
embedded_in :tag
end
article_controller.rb
class ArticlesController < ApplicationController
http_basic_authenticate_with name: "dhh", password: "secret", except: [:index, :show]
def index
#articles = Article.all
end
def show
#article = Article.find(params[:id])
end
def new
#article = Article.new
end
def edit
#article = Article.find(params[:id])
end
def create
#article = Article.new(article_params)
if #article.save
redirect_to #article
else
render 'new'
end
end
def update
#article = Article.find(params[:id])
if #article.update(article_params)
redirect_to #article
else
render 'edit'
end
end
def destroy
#article = Article.find(params[:id])
#article.destroy
redirect_to articles_path
end
private
def article_params
params.require(:article).permit(:title, :text, :all_tags)
end
end
_form.html.erb
<%= form_for #article do |f| %>
<% if #article.errors.any? %>
<div id="error_explanation">
<h2>
<%= pluralize(#article.errors.count, "error") %> prohibited
this article from being saved:
</h2>
<ul>
<% #article.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
<% end %>
<p>
<%= f.label :title %><br>
<%= f.text_field :title %>
</p>
<p>
<%= f.label :text %><br>
<%= f.text_area :text %>
</p>
<p><%= f.text_field :all_tags, placeholder: "Tags separated with comma" %></p>
<p>
<%= f.submit %>
</p>
<% end %>