The Ultimate Guide to Polymorphic Associations in Ruby on Rails

If you‘ve been working with Ruby on Rails for a while, you‘re probably familiar with the basic active record associations – belongs_to, has_one, and has_many. These relationships allow you to efficiently connect models together.

But what if you have a model that needs to belong to multiple other models? For example, say you‘re building a social media application. You have posts, photos, and videos that users can comment on. Without polymorphic associations, you‘d need a separate comments table for each of those models. This leads to duplicate code and an overly complex database schema as your application grows.

Luckily, Rails provides an elegant solution in the form of polymorphic associations. With polymorphic associations, you can connect a model to multiple other models using a single association. This keeps your code DRY and your database tidy.

In this comprehensive guide, we‘ll dive deep into polymorphic associations. You‘ll learn what they are, how they work, and see detailed code examples. By the end, you‘ll be able to efficiently implement polymorphic associations in your own Rails applications. Let‘s jump in!

What are Polymorphic Associations?

From the Rails Guides, a polymorphic association "allows a model to belong to more than one other model, on a single association."

In simpler terms, it allows a single model to connect to multiple other models without needing to define separate foreign keys for each one. The polymorphic model doesn‘t care what type of model it belongs to, as long as that model includes the necessary polymorphic interface.

Let‘s look at a concrete example to clarify. Imagine you‘re building a fitness tracking application with the following models:

class User < ApplicationRecord
  has_many :workouts
  has_many :meals
end

class Workout < ApplicationRecord
  belongs_to :user
  has_many :exercises
end 

class Meal < ApplicationRecord
  belongs_to :user
  has_many :foods
end

Now you want to add the ability for users to log notes about their individual workouts and meals. You could create separate WorkoutNote and MealNote models, each with a has_many association on Workout and Meal. But this would lead to a lot of duplicated code.

Instead, you can use a polymorphic association:

class Note < ApplicationRecord
  belongs_to :notable, polymorphic: true
end

class Workout < ApplicationRecord
  belongs_to :user
  has_many :exercises  
  has_many :notes, as: :notable
end

class Meal < ApplicationRecord
  belongs_to :user
  has_many :foods
  has_many :notes, as: :notable  
end

Now the Note model can belong to either a Workout or Meal using the single :notable association. The Workout and Meal models use has_many :notes, as: :notable to set up the other side of the relationship.

This is the power of polymorphic associations. They allow a single model to flexibly connect to multiple other models, keeping your code clean and your database streamlined.

Under the Hood

To really understand polymorphic associations, it helps to look at how they work under the hood. When you define a polymorphic belongs_to association, Rails creates two columns on the table – a foreign key column (notable_id in our example above) and a type column (notable_type).

The foreign key column, notable_id, stores the ID of the associated record, just like a normal foreign key. The type column, notable_type, stores the class name of the associated record. Rails uses this type column to know which model the ID in notable_id refers to.

When you call an association method on a polymorphic association, like note.notable, Rails looks at the notable_type column to determine which model to fetch the notable_id from. It then returns the associated record.

Here‘s what the schema might look like for our notes example:

create_table "notes", force: :cascade do |t|
  t.text "body"
  t.integer "notable_id"
  t.string "notable_type"
  t.datetime "created_at", precision: 6, null: false
  t.datetime "updated_at", precision: 6, null: false
  t.index ["notable_type", "notable_id"], name: "index_notes_on_notable"
end

Notice the notable_id and notable_type columns, as well as the composite index on both columns. This index is necessary for performance when fetching polymorphic associations.

Setting Up Polymorphic Associations

Now that you understand what polymorphic associations are and how they work, let‘s walk through setting one up step-by-step.

First, generate your polymorphic model. We‘ll call it Comment in this example:

rails generate model Comment body:text commentable:references{polymorphic}  

This will create a Comment model and a migration to create the comments table. The commentable:references{polymorphic} argument tells Rails to set up the polymorphic association.

The migration should look something like this:

class CreateComments < ActiveRecord::Migration[6.1]
  def change
    create_table :comments do |t|
      t.text :body
      t.references :commentable, polymorphic: true, null: false
      t.timestamps
    end
  end
end

Run the migration:

rails db:migrate

Next, set up the associations on your Comment model:

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
end

Finally, add the has_many associations to the models you want to be commentable:

class Post < ApplicationRecord
  has_many :comments, as: :commentable
end

class Photo < ApplicationRecord
  has_many :comments, as: :commentable  
end

class Video < ApplicationRecord
  has_many :comments, as: :commentable
end  

That‘s it! You‘re now ready to create comments that belong to posts, photos, or videos.

post = Post.first
post.comments.create(body: "Great post!")

photo = Photo.first 
photo.comments.create(body: "Nice shot!")

Rails will automatically set the commentable_id and commentable_type fields for you when you create the association.

Use Cases

Polymorphic associations are useful in a variety of scenarios. Here are some common use cases:

• Comments: As we saw in the examples above, polymorphic associations are commonly used to set up commenting systems where comments can belong to multiple types of models (posts, photos, etc).

• Likes/Favorites: Polymorphic associations are also often used to allow users to "like" or "favorite" multiple types of content. A single Like model can belong to posts, photos, videos, etc.

• Tagging: Polymorphic associations can be used to set up tagging systems where a single Tag model can connect to multiple other models. For example, you could tag blog posts, products, or user profiles.

• Activity Feeds: In social applications, polymorphic associations are often used to build activity feeds. A single Activity model might connect to many types of content (posts, comments, likes, follows, etc).

Here‘s a more complex, real-world example. Let‘s say you‘re building a project management application. You have Project, Task, and Document models. You want to allow users to leave comments on all three types.

With polymorphic associations, you can set this up cleanly:

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
  belongs_to :user
end

class Project < ApplicationRecord
  has_many :comments, as: :commentable
  has_many :tasks
  has_many :documents 
end

class Task < ApplicationRecord
  belongs_to :project
  has_many :comments, as: :commentable
end

class Document < ApplicationRecord
  belongs_to :project  
  has_many :comments, as: :commentable
end

class User < ApplicationRecord
  has_many :comments
end

This setup allows maximum flexibility. Users can leave comments on projects, individual tasks, and documents, and you can easily fetch all of a user‘s comments or all comments for a given project, task, or document.

Best Practices and Gotchas

While polymorphic associations are a powerful tool, there are a few things to keep in mind when using them.

• Naming: As you saw in the examples above, polymorphic associations use a more generic name for the association (like commentable or notable). Avoid names that are too generic, like object or thing, as they can make your code harder to understand.

• Indexing: Always add indexes on the polymorphic columns (commentable_id and commentable_type in our examples). Rails does this automatically when you use the references keyword in your migration. Without these indexes, querying polymorphic associations can be slow.

• Eager Loading: When you‘re working with polymorphic associations, eager loading can be a bit tricky. You can‘t just use includes like you normally would. Instead, you need to specify the models:

  Comment.includes(commentable: [:post, :photo, :video])

  This will eagerly load the associated post, photo, and video records for each comment.

• Dependent Destruction: If you want associated records to be deleted when the parent record is deleted (e.g., deleting a post deletes all its comments), you need to specify dependent: :destroy on the has_many side of the association:

  class Post < ApplicationRecord
    has_many :comments, as: :commentable, dependent: :destroy
  end

• Existence Checks: When you have a polymorphic belongs_to association, Rails doesn‘t automatically validate the existence of the associated record. If you need this validation, you can add it manually:

  class Comment < ApplicationRecord
    belongs_to :commentable, polymorphic: true
    validates :commentable, presence: true
  end  

Alternatives to Polymorphic Associations

While polymorphic associations are a great solution for many use cases, they‘re not always the best choice. One common alternative is Single Table Inheritance (STI).

With STI, you use a single table to represent multiple models. For example, instead of having separate Post, Photo, and Video models, you might have a single Content model with a type column. The type column would contain the class name (Post, Photo, or Video), and Rails would automatically instantiate the correct model based on this value.

STI works well when your models are very similar and share most of their attributes and behavior. However, it can become cumbersome if your models start to diverge significantly.

Another alternative is to simply use separate tables and associations. If you only need to connect two models, or if the associations are very different from each other, separate associations may be clearer and simpler than a polymorphic association.

Performance Considerations

While polymorphic associations are very convenient, they can come with some performance costs.

Because polymorphic associations use two columns (commentable_id and commentable_type in our examples), they require an extra join when fetching associated records. This can slow down queries, especially on large tables.

Moreover, because the associated records can be of different types, it‘s not always possible to optimize these queries with indexing or other techniques.

That said, for most applications, the performance impact of polymorphic associations will be negligible. They‘re a widely used feature of Rails and are well-optimized. However, if you‘re dealing with very large scale or performance-critical parts of your application, it‘s worth benchmarking and considering alternatives.

Conclusion

Polymorphic associations are a powerful feature of Ruby on Rails that allow a single model to belong to multiple other models. They‘re commonly used for features like comments, likes, favorites, and tagging, where a single interaction can be applied to many different types of content.

Setting up polymorphic associations is straightforward. On the polymorphic model, you define a belongs_to association with the polymorphic: true option. On the associated models, you define a has_many association with the as: :association_name option.

While polymorphic associations are very useful, they‘re not always the best solution. For models that are very similar, Single Table Inheritance (STI) may be a better choice. And for simple one-to-one connections, separate associations may be clearer.

As with any powerful abstraction, polymorphic associations come with some potential gotchas and performance considerations. However, for most applications, these are minor concerns outweighed by the flexibility and maintainability benefits of polymorphic associations.

By understanding how polymorphic associations work and when to use them, you can keep your Rails code clean, expressive, and maintainable as your application grows in complexity. Happy coding!