Scope vs Class Methods

Recently in one of interviews, I was asked about this and that led me to revisit these concepts again.

What is Scope in Ruby on Rails?

In Ruby on Rails, scope refers to a way to define commonly used queries that can be referenced as method calls on ActiveRecord models. Scopes allow you to encapsulate query logic, making it reusable and easy to maintain.

Example scope of Post model, in which we frequently need to fetch published posts, we can define a scope like so:

class Post < ApplicationRecord
  scope :published, -> { where(published: true) }
end

Characteristics of Scopes

• Chainable: Since scopes always return an ActiveRecord::Relation, they're guaranteed to be chainable with other scopes or query methods.
• Simplified Syntax: Scopes provide a more succinct way to write reusable query logic with a clear intention.
• Default Scope: You can define a default scope that applies to all queries on the model.

What are Class Methods in Ruby on Rails?

Class methods in Ruby on Rails serve a similar purpose as scopes, allowing you to define methods at the class level. These methods can perform operations or queries related to the class they are defined in.

Example of class method on Post model, we can define a class method to achieve the same result as the scope:

class Post < ApplicationRecord
  def self.published
    where(published: true)
  end
end

Characteristics of Class Methods

• Flexibility: Class methods can contain any Ruby code, which allows for complex logic that might be awkward or impossible to define in a scope.
• Return Values: While they can return an ActiveRecord::Relation to allow for chaining, they can also return any other type of object, depending on the method's internal logic.

Both of these methods can be called Post. self.published

When to Use Each

The choice between using a scope and a class method boils down to the complexity of the logic you need to implement and personal preference. Here are some guidelines:

• Use scopes for simple, straightforward query conditions that will always return an ActiveRecord::Relation. They're more readable and directly convey the intention of returning a query result.
• Use class methods when your query logic is complex, requires conditional logic, or when you might need to return something other than an ActiveRecord::Relation. Class methods offer the full power of Ruby, making them suitable for scenarios where scopes might not be sufficient.

In practice, many Rails developers use a combination of both, leveraging scopes for the majority of straightforward query needs and reserving class methods for when those needs exceed the neat encapsulation scopes offer. This approach takes advantage of the strengths of both techniques, leading to more maintainable and understandable model code.