collection_select doesn't save through form - ruby-on-rails

Question

I know there is a lot of similar posts where people have the same problem, but none of them helped me. If i create new article then it won't have a category. But if i edit article created earlier in seed.rb then the category is updated.
What's wrong?
Categories table:
class CreateCategories < ActiveRecord::Migration
def change
create_table :categories do |t|
t.string :name
t.timestamps
end
end
end
category.rb
class Category < ActiveRecord::Base
has_many :articles
end
article.rb
class Article < ActiveRecord::Base
belongs_to :category
end
and then i have a _form file
<%= form_for(#article) do |f| %>
<div class="title">
<%= f.label :title %>
<%= f.text_field :title %>
</div>
<div class="content">
<%= f.label :content %>
<%= f.text_field :content %>
</div>
<div class="category">
<%= f.label :category %>
<%= collection_select(:article, :category_id, Category.all, :id, :name) %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
There is a categories_controller file:
class CategoriesController < ApplicationController
def index
#categories = Category.all
end
def show
#category = Category.find(params[:id])
#articles = #category.articles
end
def new
#category = Category.new
end
def create
#category = Category.new(category_params)
if #category.save
redirect_to(:action => 'index')
else
render('new')
end
end
def edit
#category = Category.find(params[:id])
end
def update
#category = Category.find(params[:id])
if #category.update_attributes(category_params)
redirect_to(:action => 'show', :id => #category.id)
else
render('index')
end
end
def delete
#category = Category.find(params[:id])
end
def destroy
Category.find(params[:id]).destroy
redirect_to(:action => 'index')
end
private
def category_params
params.require(:category).permit(:name)
end
end
Articles controller file:
class ArticlesController < ApplicationController
def index
#articles = Article.all
end
def show
#article = Article.find(params[:id])
end
def new
#article = Article.new
end
def create
#article = Article.new(article_params)
if #article.save
redirect_to(:action => 'index')
else
render('new')
end
end
def edit
#article = Article.find(params[:id])
end
def update
#article = Article.find(params[:id])
if #article.update_attributes(article_params)
redirect_to(:action => 'show', :id => #article.id)
else
render('index')
end
end
def delete
#article = Article.find(params[:id])
end
def destroy
Article.find(params[:id]).destroy
redirect_to(:action => 'index')
end
private
def article_params
params.require(:article).permit(:title, :content)
end
end

Answer 1

It looks like you're not building the association in the articles#create action. The category_id is being sent through your form, but you still need to to build the Active Record association. You could try something like this in the articles controller:
def create
#article = Article.new(article_params)
#category = Category.find(params[:category_id])
#article.category = #category
if #article.save
redirect_to(:action => 'index')
else
render('new')
end
end
Keep in mind, there are multiple ways to create an association. Your Article class has the following five methods to manipulate the association:
#article.category
#article.category=
#article.build_category
#article.create_category
#article.create_category!