Join table for has_many through in Rails

Question

I am new to programming & rails and there's something I dont fully understand. I am creating an app with

product has_many categories
category has_many products

If I understand correctly I need to create a join table products_categories which has a product_id & a category_id. First do I also need a model for this table ? if yes I guess it would look like this :

class CategoryProduct < ActiveRecord::Base
   belongs_to :category
   belongs_to :product
end

and the other models in product.rb :

 class Product < ActiveRecord::Base
  has_many :category_products
  has_many :categories, through: :category_product
  has_attached_file :picture,
    styles: { medium: "300x300>", thumb: "100x100>" }

  validates_attachment_content_type :picture,
    content_type: /\Aimage\/.*\z/
  validates :price,               presence: { message: "Merci d'indiquer le prix du produit" }
  validates :name,                presence: { message: "Merci d'indiquer le nom du produit" }
  validates :weight,              presence: { message: "Merci d'indiquer le poids du produit" }
  validates :description,         presence: { message: "Merci d'écrire une description du produit " }
end

and in category.rb

    class Category < ActiveRecord::Base
  has_many :category_products
  has_many :products,         through: :category_product
  validates :name,                presence: { message: "Merci d'indiquer le nom de la catégorie" }
end

Now let's say I want to create a product and while I am creating it, specify as many categories as I want for this product from the categories' list.

So far this was my Product/new.html.slim in my views :

  div class="container marged-top"
    div class= "col-xs-12 col-md-offset-3 col-md-5 bigmarge"
      div class="panel panel-default"
        div class= "panel-heading"
          h4 Création Produit
        div class= "panel-body"
          =simple_form_for @product, html: { multipart: true } do |t|
            = t.error_notification
            = t.input :name, label: 'Nom'
            = t.input :description, label: 'Description', required: true
            = t.input :price, label: 'Prix', required: true
            = t.input :weight, label: 'Poids', required: true
            = t.label :picture
            = t.file_field :picture
            = t.association :categories, as: :check_boxes
            = t.button :submit, value: "Valider",  class: "btn-success marge-bas"

This was a simple form for my Product instance. I guess I need to have a form for a CategoryProduct now ? How do I have to change that if I want the user to be able to add as many categories he wants to the product while he's creating it ?

Here's my migration file for category_product table :

class CreateTableCategoriesProducts < ActiveRecord::Migration

  def change
    create_table :categories_products do |t|
      t.references :product, index: true
      t.references :category, index: true
    end
    add_foreign_key :categories_products, :categories
    add_foreign_key :categories_products, :products
  end
end

I renamed the previous table with the following migration file :

class RenameTableCategoriesProducts < ActiveRecord::Migration
  def self.up
    rename_table :categories_products, :category_products
  end

 def self.down
    rename_table :category_products, :categories_products
 end
end

I am getting the following error on the simple_form in product/new.html.slim :

undefined method `klass' for nil:NilClass

The code breaks here :

 = t.association :categories, as: :check_boxes

so I guess my associations arent still quite right

Answer 1

In Rails there are two ways to do many-to-many relationships:

has_and_belongs_to_many

sets up a many to many relationship without an intervening model.

class Category < ActiveRecord::Base
  has_and_belongs_to_many :products
end

class Product < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

This is nice if you know that it is a simple direct relationship and you know that you will not need to store any additional data about the relationship. It uses less memory since it does not have to instantiate an extra model just to do product.category.

When using has_and_belongs_to_many the convention is that the join table is named after the two entities in plural. In alfabetical order:

Category + Product = products_categories

has_many through

as you already have guessed uses an intermediate model.

class CategoryProduct < ActiveRecord::Base
  belongs_to :product
  belongs_to :category
end

class Category < ActiveRecord::Base
  has_many :category_products
  has_many :products, through: :category_products
end

class Product < ActiveRecord::Base
  has_many :category_products
  has_many :categories, through: :category_products
end

The advantage here is that you can store and retrieve additional data in the join table which describes the relationship. For example if you wanted to store who added a product to a category - or when the relationship was created.

In order to for Rails to be able to correctly find the ProductCategory class the join table for a has_many though naming convention is

model 1(singular) + model 2(plural) 
Product + Category = category_products

This is due to the way that rails infers the model class based on the table name. Using categories_products would case rails to look for Category::CategoriesProduct.

Many to Many in forms and controllers.

As IvanSelivanov already mentioned SimpleForm has helper methods for creating selects, checkboxes etc.

• https://github.com/plataformatec/simple_form#associations

But instead of overriding the .to_s method in your model you may want to use the label_method option instead.

f.assocation :categories, as: :checkboxes, label_method: :name

Overriding .to_s can make debugging harder and in some cases give confusing test error messages.

To whitelist the params in your controller you would do:

class ProductsController < ApplicationController
  def create
    @product = Product.new(product_params)
    if @product.save
      redirect_to @product
    else
      render :new
    end
  end

  def product_params
     params.require(:product)
           .permit(:name, :categories_ids, ...)
  end
end

Answer 2

You also have to add CategoryProduct to each model:

class Product < ActiveRecord::Base
  has_many :category_products
  has_many :categories, through: :category_product

It is very simple using gem simple form. All you have to do is to add:

t.association :categories

in a form for product and add :category_ids => [] to a list of permitted parameters in your products controller

If you prefer checkboxes instead of multi-select list, you can do

    t.association :categories, as: check_boxes

And the last thing, to display categories in human-readable format, you need to define a to_s method in your category model, i. e.:

class Category < ActiveRecord::Base
  ...
  def to_s
    name
  end 
end