A Concern is a module that you extract to split the implementation of a class or module instead of having a big class body.
A Rails concern is just a plain Ruby module that extends the ActiveSupport::Concern module provided by Rails.

Key Characteristics of Concerns:

• Modularity: Concerns break down complex classes into similar, more focused units, improving code readability and understanding.

• Reusability: A concern can be reused across multiple classes, reducing code duplication and promoting DRY principle.

• Maintainability: Concerns can be easily extended or customized to fit specific use cases, offering flexibility and adaptability.

• Testing: Concerns can be tested independently, ensuring their correctness and reliability.

Creating a concern:

1. Create a new Ruby module file in your app/controllers/concerns

2. Extend the ActiveSupport::Concern module to make it a Rails concern.

3. Define methods or class methods that encapsulates the desired functionality

4. Use the included block to automatically include the concern's methods in classes that include it.

Example

# app/controllers/concerns/searchable.rb
module Searchable
  extend ActiveSupport::Concern

  included do
    scope :search, ->(query) { where("name ILIKE ?", "%#{query}%") }
  end
end

Example using a Concern:

class Product < ApplicationRecord
  include Searchable
end