Simplifying Polymorphic Associations with Rails ActiveRecord::DelegatedType
Published: 2024-01-11 17:07:45

Hey Rails dev! Ever struggled with handling different types of data in your Rails applications? Well, `ActiveRecord::DelegatedType` is here to make your life easier. In this guide, I'll walk through how you can use `DelegatedType` to manage diverse data relationships in a more organized way. It's like having a neat filing system for your database tables. So, let's dive in and explore how `DelegatedType` can help you keep your Rails projects well-structured and easy to maintain.

## What!
Let's introduce ActiveRecord's `DelegatedType`. 

It is a Rails pattern for managing polymorphic associations in a more structured and efficient way. Think of it as a way to store a superclass that can be subclassed using delegation instead of inheritance. It's like having a Swiss Army knife in your Rails ActiveRecord toolkit!

## When to use `DelegatedType`?
Leverage `DelegatedType` when you need to:
- Manage objects sharing common features and behaviours but requiring distinct handling.
- Simplify complex relationships under a unified interface.

## How: the core idea
At its core, `DelegatedType` simplifies the handling of models that can represent multiple types of related data without the complexity of Single Table Inheritance (STI).

For example, In a blog application with posts, comments, and authors, `ActiveRecord::DelegatedType` is very handy when you want to have a single model that can represent different types of related data.

## Why, embrace `DelegatedType`?
Here’s why `DelegatedType` can be your best friend:
- **Simplifies Polymorphic Relationships:** Manages different types of data under a unified interface.
- **Avoids STI Drawbacks:** Prevents cluttering a single table with columns relevant to only specific types.
- **Enhances Code Readability:** Clear separation of concerns, making it easier to understand and maintain.

## Example scenario, Blog application
Consider a blog application with posts, comments, and authors, where posts and comments share common features but also have unique attributes.
### Step-by-Step Implementation

#### 1. Define a Common Interface
Create a module, say `Publishable`, to encapsulate shared logic and behaviour.

```ruby
# app/models/concerns/publishable.rb
module Publishable
  extend ActiveSupport::Concern

  included do
    has_one :publication, as: :publishable, touch: true
    # Other shared logic...
  end
end
```

#### 2. Create a Superclass Model
Define a `Publication` model to act as a superclass using `delegated_type`.

```ruby
# app/models/publication.rb
class Publication < ApplicationRecord
  delegated_type :publishable, types: %w[ Post Comment ]
  
  belongs_to :author
  # Common logic like publication date and author references
end
```

#### 3. Define Subclasses
Implement `Post` and `Comment` models, including the `Publishable` module.

```ruby
class Post < ApplicationRecord
  include Publishable
  # Attributes specific to posts
end

class Comment < ApplicationRecord
  include Publishable
  # Attributes specific to comments
end
```

#### 4. Handling Records
Create `Publications` that are either `Posts` or `Comments`.

```ruby
# Creating a new post
Publication.create!(publishable: Post.new(title: 'My First Post'), author: some_author)

# Creating a new comment
Publication.create!(publishable: Comment.new(content: 'Great post!'), author: another_author)
```

Or generate [seed data](https://github.com/AhmedNadar/delegate_blog/blob/main/db/seeds.rb).
## Advanced Use Cases

### Nested Attributes
You can use nested attributes to handle form submissions for both posts and comments through the `Publication` model.

```ruby
class Publication < ApplicationRecord
  accepts_nested_attributes_for :publishable
  # ...
end
```

### Delegating Methods
Delegating methods to the subclasses can streamline accessing subclass-specific attributes.

```ruby
class Publication < ApplicationRecord
	# Delegating a method (e.g., title for Post, content for Comment) to the publishable object
	delegate :title, :content, to: :publishable, allow_nil: true
end
```

### Querying
Querying is straightforward; you can fetch either all publications, just posts, or just comments.

```ruby
# Fetch all publications
publications = Publication.all

# Fetch all posts
posts = Publication.posts

# Fetch all comments
comments = Publication.comments
```

## 📋 Conclusion
As we have seen `ActiveRecord::DelegatedType` offers a robust and flexible way to handle polymorphic associations. You can effectively implement and leverage this pattern, enhancing the structure and maintainability of your Rails applications. 

## 💻 Demo repo
Now, curious to see `ActiveRecord::DelegatedType` in action? I've created a demo Rails app that showcases the usage of `DelegatedType` for managing polymorphic associations, along with advanced use cases. In addition, the app includes views to demonstrate the display of all posts and publications, making it a handy tool for app admins who need to oversee various data objects at once. You can check out the demo and explore the code on [GitHub](https://github.com/AhmedNadar/delegate_blog). It's a great way to get a hands-on understanding of how `DelegatedType` can streamline your data management tasks.

## 📚 Further Learning
For more in-depth examples and advanced use cases, check [Rails DelegatedType API documentation](https://edgeapi.rubyonrails.org/classes/ActiveRecord/DelegatedType.html).