Yassir Hartani

Yassir Hartani

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Understanding has_many through in rails

If you are struggling to understand the has_many through in rails, I am confident that these three sections will help you have a clear idea on why we use and when to use this helper.

  1. Understanding the fundamentals of the many to many relationship in the relational database management system(RDMB) specially the join table.

  2. We’ll Create an example where we’ll add basic functionalities(Create/Delete) without using the has_many through.

  3. Then add the helper and refactor the code.

Understanding the many to many relationship

Let’s take an Ecommerce business as an example where we have products and orders.

A user can place an order with many products, and a product can be in many orders, so it’s a many to many relationship between the order and the product.

Now let’s represent this relationship.

In the RDBM the only way to represent this is by creating a new table(Joining table or Join table), this table will have two columns the order_id and the product_id. Let’s name this table order_product (It’s a convention to name the table by combining the two other table names).

table diagram

To visualize how this relationship works, let’s make a small example. A customer created an order with 4 products, order_id is 3 and products_ids are (1 | 2 | 3 |4).

In our order_product table we will insert 4 rows with order_id of 4 and with the relevant product_id.

order_product table diagram

Now when we’ll query the order products, we have to filter the products based on the order_id = 3.

To learn more about the many to many relationship, I encourage you to read this article. https://dzone.com/articles/how-to-handle-a-many-to-many-relationship-in-datab

Now let’s Code it.

Create a Rails project with the models and associations

In this section, we’ll create a rails project and generate three models order, product, and the join table order_product.

First, go and create a new rails project, when the project is created let’s generate the models.

First, go and create a new rails project, when the project is created let’s generate the models.

rails generate model order status:string user:references
rails generate model product name:string price:decimal
rails generate model order_product order:references product:references

Finally, run rails db:migrate to create our tables in the DB.

Remember that the main association is between products and orders by using the join table order_product, so the orders and products will have many order_product, but the order_product model will belong to both orders and products. Let’s add it!

class Order < ApplicationRecord
   has_many :order_product
class Product < ApplicationRecord
   has_many :order_product
class OrderProduct < ApplicationRecord
 belongs_to :order
 belongs_to :product

Now let’s Jump into the command line and start adding functionalities. Open the console rails console and first let’s generate some data

50.times do
   Product.create(name: “product_name”, price: 10)

10.times do
   order = orders.create()
   5.times do
       OrderProduct.create(order_id: order.id, product_id: Product.find(rand(1...50).id))

This will generate 50 products and 10 orders each order with 5 products.

Adding basic functionalities without has_many through

The functionalities that we’ll add are

  • Query all the products in an order
  • Adding a product to an order

Query all the products from the order

first, let’s get the order id we want to query the products from

order_id = Order.first.id

Then we will query the data from the OrderProduct by passing the order_id and mapping through the data to return just the product.

OrderProduct.where(order_id: order_id).map {|item| item.product }

Adding a product to an order

order_id = Order.first.id
product_id = Product.find(20)

OrderProduct.create(order_id: order_id, product_id: product_id)

Whenever we want to query or insert a product we need to use the OrderProduct model and pass the required parameters, when using the has_many through method we won’t need to use it anymore and the code will be much simpler and cleaner.

Using the has_many through

Let’s add the method to model/order.rb

class Order < ApplicationRecord
   has_many :order_product
   has_many :products, through: :order_product

Let’s jump to the rails console again(refresh the rails console using reload! command.)

Query all the products from the order


Adding a product to an order

Order.first.products << Product.find(22)

Sweet, no more interacting with the join model.


The rails documentation is the best place to learn, but sometimes to understand the use of a certain method you must create an example with and without the method this way you will have a deep knowledge than just reading about it.

Thanks for reading !!

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