Update on Rails 4 using :has_many :through many relationship for many (has_and_belongs_to_many)

Question

I’m having a question about how to update Rails 4 using :has_many :through, if there is a more correct and easier way, I am open to suggestions.

Error:

app/views/Articles/_form.html.erb

 <label>Category</label>
  <% @categories.each do |category| %>
  <div class="field">
    <%= check_box_tag "categories[]", category.id, 
                        @article.category_ids.include?(category.id) %> 
    <%= h category.name %>
  </div>
  <% end %>

app/controllers/articles_controller.Rb

class ArticlesController < ApplicationController
  before_action :set_article, only: [:show, :edit, :update, :destroy]
  before_action :set_category, only: [:show, :edit, :update, :new]
  # GET /articles
  # GET /articles.json
  def index
    @articles = Article.all
  end

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

  # GET /articles/new
  def new
    @article = Article.new
  end

  # GET /articles/1/edit
  def edit
  end

  # POST /articles
  # POST /articles.json
  def create
    @article = Article.new(article_params)
    @article.categories = params[:categories]
    #raise params.to_yaml
    respond_to do |format|
      if @article.save
        format.html { redirect_to @article, notice: 'Article was successfully created.' }
        format.json { render :show, status: :created, location: @article }
      else
        format.html { render :new }
        format.json { render json: @article.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /articles/1
  # PATCH/PUT /articles/1.json
  def update
    respond_to do |format|
      if @article.update(article_params)
        @article.categories_up = params[:categories]
        #params[:article][:category_ids] ||= []
        format.html { redirect_to @article, notice: 'Article was successfully updated.' }
        format.json { render :show, status: :ok, location: @article }
      else
        format.html { render :edit }
        format.json { render json: @article.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /articles/1
  # DELETE /articles/1.json
  def destroy
    @article.destroy
    respond_to do |format|
      format.html { redirect_to articles_url, notice: 'Article was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

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

    # Never trust parameters from the scary internet, only allow the white list through.
    def article_params
      params.require(:article).permit(:title, :body, :categories)
    end

    def set_category
      @categories = Category.all
    end
end

app/models/article.Rb

class Article < ActiveRecord::Base
    has_many :has_categories
    has_many :categories, :through  => :has_categories
    after_save :save_categories
    after_update :update_categories_up

    # custom setter method
    def categories=(value)
        @categories = value
    end

    # custom setter method
    def categories_up=(value)
        @categories = value
    end

    private

    def save_categories
        #raise @categories.to_yaml
        @categories.each do |category_id|
            HasCategory.create(category_id: category_id, article_id: self.id)
        end
    end

    def update_categories_up
        #raise @categories.to_yaml
        @categories.each do |category_id|
            HasCategory.update(category_id: category_id, article_id: self.id)
        end
    end

Answer 1

You have a callback calling the method Article#update_categories_up. This method is called at the controller line that calls the Article#update (if @article.update(article_params)). At that time the @categories in its object Article is nil because it was not set (you arrow it right after). Try setting the categories_up= before calling the update.

Another tip is to avoid using callbacks, they make code confusing and difficult to debug.