Active Record associations can be used to describe one-to-one, one-to-many and many-to-many relationships between models. Each model uses an association to describe its role in the relation. The belongs_to association is always used in the model that has the foreign key.

(1) One-to-one

Use has_one in the base and belongs_to in the associated model.

models/employee.rb
has_one :address

models/address.rb
belongs_to :employee #foreign key – employee_id
end

(2) One-to-many

Use has_many in the base, and belongs_to in the associated model.

models/user.rb
has_many :events

models/event.rb
belongs_to :user #foreign key – user_id

(3) Many-to-Many

There are two different ways to set up a many-to-many association in Rails.

1. has_and_belongs_to_many (often reffered to as habtm)

In migration
def self.up

create_table ‘categories_products’, :id => false do |t| t.column :category_id, :integer t.column :product_id, :integer end end

models/product.rb
has_and_belongs_to_many :categories

models/category.rb
has_and_belongs_to_many :products

Here join table ‘categories_products’ contains primary keys of both the tables. The table name follows alphabetical order of each table, separated by an underscore.

2. has_many :through

models/categorization.rb
belongs_to :product
belongs_to :category

models/product.rb
has_many :categorizations
has_many :categories, :through => :categorizations

models/category.rb
has_many :categorizations
has_many :products, :through => :categorizations

This technique is used when you want to put some additional fields in median table. Here categorization is a full model that represent the join table.

So, with these tips you can choose the right one for your project.