What is the use of join table in Rails?

Question

For what purposes I'd potentially need to use join table?

For example, when I run rails g migration CreateJoinTable products suppliers, it creates the respective products_suppliers table with products_id and suppliers_id columns. Moreover, these fields have the option :null set to false by default. Why are those fields needed? What they are used for? And what is that null option?

Thanks in advance.

Answer 1

It's standard relational database functionality.

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Rails is designed on top of a relational database (typically MYSQL or PGSQL), which basically means that you're able to reference "associated" data through the use of foreign keys:

In context of relational databases, a foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table

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In the case of Rails, "relationships" in the database are maintained by ActiveRecord - an ORM (Object Relational Mapper). This means that from the application layer, you just have to focus on populating objects:

@user = User.find x
@user.products #-> outputs records from "products" table with foreign key "user_id = x"

ActiveRecord manages your associations, which is why you have to define the belongs_to / has_many directives in your models etc.

Most associations you create will be reference other tables directly:

#app/models/user.rb
class User < ActiveRecord::Base
   has_many :products
end

#app/models/product.rb
class Product < ActiveRecord::Base
   belongs_to :user
end

The problem with this is that it only allows you to associate single records; if you wanted to associate multiple (many-to-many), you need a join table.

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Rails uses join tables for has_many :through and has_and_belongs_to_many relationships...

Join tables are populated with (at least) the primary key & foreign key of a relationship. For example...

user_id | product_id
   1    |     3
   1    |     5
   2    |     3

This allows you to call:

#app/models/user.rb
class User < ActiveRecord::Base
   has_and_belongs_to_many :products
end

#app/models/product.rb
class Product < ActiveRecord::Base
   has_and_belongs_to_many :users
end

@user.products #-> all products from join table
@product.users #-> all users from join table

In short, if you want to have has_many <-> has_many associations, the join table is necessary to store all the references to the relative foreign keys.