Differences Between has_many :through and has_and_belongs_to_many in Ruby on Rails

In Ruby on Rails, when you need to establish many-to-many relationships between models, two primary associations come into play: has_many :through and has_and_belongs_to_many (often abbreviated as HABTM). Each has its own use cases and advantages, but the choice between them depends on your specific project requirements. Here are some of the key differences: 

1. Join Model:

has_and_belongs_to_many (HABTM) doesn't create a separate model for the join table. It relies on a simple join table with no additional attributes.
has_many :through allows you to create a separate model for the join table. This join model can have its own attributes and methods.

2. Flexibility:

has_and_belongs_to_many is simpler and more suitable when you only need a basic many-to-many association without additional data on the join table.
has_many :through offers more flexibility, making it suitable when you need to store extra information or logic related to the association.

3. Table Naming:

has_and_belongs_to_many relies on a naming convention for the join table, combining the names of the associated models in alphabetical order.
has_many :through allows you to specify the name of the join table explicitly.

4. Accessing Data:

With has_and_belongs_to_many, you can access the associated records directly through the models.
has_many :through provides a more explicit way to access the associated records by going through the join model.

has_many :through Example:

Let's say we have two models, User and Course, and we want to establish a many-to-many relationship between them using has_many :through. Here's how you would set it up:

# User model
class User < ApplicationRecord
  has_many :user_courses
  has_many :courses, through: :user_courses
end

# Course model
class Course < ApplicationRecord
  has_many :user_courses
  has_many :users, through: :user_courses
end

# UserCourse model (join model)
class UserCourse < ApplicationRecord
  belongs_to :user
  belongs_to :course
end

In this example, we create a join model called UserCourse to store additional information related to the relationship, such as enrollment date or grades. The has_many :through association allows us to access courses through users and vice versa.

has_and_belongs_to_many Example:

Now, let's look at an example using has_and_belongs_to_many for the same User and Course models:

# User model
class User < ApplicationRecord
  has_and_belongs_to_many :courses
end

# Course model
class Course < ApplicationRecord
  has_and_belongs_to_many :users
end

In this case, we don't need a separate join model. Rails will create a join table named courses_users by convention to store the relationships. This approach is simpler when you don't need to store additional data about the relationship.


In summary, both has_many :through and has_and_belongs_to_many have their places in Ruby on Rails, and the choice comes down to the complexity and flexibility your project demands. Understanding the needs of your application and the nature of your relationships will guide you to the most suitable association type.