Why You Shouldn‘t Use has_and_belongs_to_many in Rails

When building associations between models in a Rails application, you‘ll often need to model a many-to-many relationship. For example, a blog application may have an association between Articles and Tags, where an article can have many tags and a tag can belong to many articles.

Rails provides two ways to implement many-to-many associations: has_and_belongs_to_many (HABTM) and has_many :through. While HABTM may seem simpler at first glance, it comes with some significant limitations that make has_many :through a better choice in most cases. Let‘s take a closer look at HABTM and explore why you should generally avoid using it in your Rails applications.

What is has_and_belongs_to_many?

The has_and_belongs_to_many association allows you to directly connect two models using a join table, without the need for an intervening join model. Here‘s an example of using HABTM to associate Articles and Tags:

class Article < ApplicationRecord
  has_and_belongs_to_many :tags
end

class Tag < ApplicationRecord
  has_and_belongs_to_many :articles
end

With this setup, Rails expects there to be an articles_tags join table to store the associations between articles and tags. The join table should have two columns: article_id and tag_id, and no primary key.

Under the hood, HABTM creates methods on each model to allow you to associate and query the related records. For example, you can do things like article.tags to get an article‘s associated tags, or tag.articles << article to associate an article with a tag.

Limitations of has_and_belongs_to_many

While HABTM provides a quick way to set up a many-to-many association, it comes with several significant limitations:

1. No join model: HABTM does not create a join model, only a join table. This means you can‘t add any additional attributes, validations, or callbacks on the association itself. If you later need to store extra data about the association or add behavior to it, you‘ll have to migrate to a has_many :through setup.

2. Lack of querying flexibility: Since there‘s no join model, you can‘t query or interact with the join table directly. This can make it harder to write certain types of queries or eager load the associated data.

3. Inflexible naming: HABTM automatically generates the name of the join table based on the alphabetical order of the model names. You have no control over the naming of the table or its columns. This can lead to awkward or unclear naming in some cases.

Here‘s an example illustrating some of these limitations:

# With HABTM, we can‘t add extra attributes to the join:
class Article < ApplicationRecord
  has_and_belongs_to_many :tags
end

class Tag < ApplicationRecord
  has_and_belongs_to_many :articles
end

# Later on, if we want to add a "created_at" timestamp to the 
# taggings, we‘d have to migrate to has_many :through

# With HABTM, we also can‘t query the join table directly:
# This won‘t work:
Tagging.where(created_at: Date.today) 

# We‘d have to do something like this instead:
Article.joins(:tags).where(articles_tags: { tag_id: tag.id })

As you can see, HABTM‘s lack of a join model limits our ability to evolve the association over time or fully control how it works.

Benefits of has_many :through

The has_many :through association solves the limitations of HABTM by using an explicit join model to connect the two main models. Here‘s how you‘d set up the Article/Tag association with has_many :through:

class Article < ApplicationRecord
  has_many :taggings
  has_many :tags, through: :taggings
end

class Tag < ApplicationRecord
  has_many :taggings
  has_many :articles, through: :taggings
end

class Tagging < ApplicationRecord
  belongs_to :tag
  belongs_to :article
end

With this setup, we have an explicit Tagging join model that connects Articles and Tags. This unlocks a number of benefits:

1. Flexibility to add attributes and behavior: Since we have a full-fledged Tagging model, we can easily add extra attributes to it, as well as validations, callbacks, and custom methods. This allows us to evolve the association over time as requirements change.

2. Ability to query the join model: We can interact with the Tagging model directly to write queries or eager load associated data. This gives us much more control and flexibility compared to HABTM.

3. Control over naming: With has_many :through, we have full control over the naming of the join table and its columns. We can choose a meaningful name like "taggings" instead of the default "articles_tags".

4. Clearer data model: Having an explicit join model makes the associations between Article, Tag, and Tagging very clear when looking at the model code. With HABTM, the association is more hidden.

Here‘s an example showing some of these benefits:

# With has_many :through, we can add attributes to the join:
class Tagging < ApplicationRecord
  belongs_to :article
  belongs_to :tag

  validates :tag_id, uniqueness: { scope: :article_id }

  scope :recent, -> { where("created_at > ?", 1.week.ago) }
end

# We can also query the join model directly:
Tagging.recent.where(article_id: article.id)

# And we have full control over naming:
class Tagging < ApplicationRecord
  # ...
  self.table_name = "article_tags"
end

As you can see, has_many :through provides a much more flexible and clear way to model many-to-many associations compared to has_and_belongs_to_many.

Why You Should Avoid HABTM

In general, it‘s recommended to avoid using has_and_belongs_to_many, even for simple many-to-many associations that don‘t currently need any extra attributes or behavior. The main reason is that requirements often change over the life of an application, and what starts as a simple association may later need to be extended in some way.

By using has_many :through from the beginning, you give yourself the flexibility to easily add attributes and behavior to the join model later on, without having to change your database schema or refactor a bunch of code. With HABTM, you‘d have to generate a migration to add the join table, create a new join model, and update all your existing association calls to use has_many :through instead.

There are a few cases where has_and_belongs_to_many might be appropriate, such as:

• You have a read-only, legacy database with an existing join table that you can‘t modify
• You‘re quickly prototyping something and don‘t care about future flexibility
• The association is guaranteed to never need any extra attributes or behavior

However, in the vast majority of cases, has_many :through is the better choice for modeling many-to-many associations in Rails.

Converting from HABTM to has_many :through

If you do have an existing has_and_belongs_to_many association that you need to convert to has_many :through, the process is fairly straightforward:

1. Generate a migration to create the join table:

rails generate migration CreateTaggings article:references tag:references

2. Create the join model:

# app/models/tagging.rb
class Tagging < ApplicationRecord
  belongs_to :article
  belongs_to :tag
end

3. Update the main models to use has_many :through:

class Article < ApplicationRecord
  has_many :taggings
  has_many :tags, through: :taggings
end

class Tag < ApplicationRecord
  has_many :taggings  
  has_many :articles, through: :taggings
end

4. Deploy the changes and run the migration.

5. Update any existing queries or association calls to use the new Tagging model.

After going through these steps, you‘ll have successfully converted your HABTM association to has_many :through and can start taking advantage of the additional flexibility and control it provides.

Conclusion

While has_and_belongs_to_many may seem like a simpler way to set up many-to-many associations in Rails, it comes with a number of significant limitations around flexibility, query power, and naming. The has_many :through association solves these issues by using an explicit join model, which allows you to add additional attributes and behavior, query the join table directly, and control the naming.

In general, it‘s best to avoid using HABTM and instead use has_many :through for your many-to-many associations. Even if your current requirements don‘t call for the extra features of has_many :through, using it from the start will give you more flexibility to extend and evolve the association later on as needs change.

With a solid understanding of the differences between these two approaches, you can make an informed decision about which one to use in your own Rails applications. By choosing has_many :through, you‘ll be setting yourself up for long-term maintainability and success.