Scopes trong Active Record là gì? Khi nào dùng scope thay vì class method?

Scope là named query được định nghĩa trong model, trả về ActiveRecord::Relation → chainable.

class Article < ApplicationRecord
  scope :published, -> { where(status: "published") }
  scope :recent,    -> { order(created_at: :desc).limit(10) }
  scope :by_tag,    ->(tag) { joins(:tags).where(tags: { name: tag }) }
end

Article.published.recent
Article.published.by_tag("ruby")

Scope vs class method: scope luôn trả về ActiveRecord::Relation (kể cả khi condition false → trả toàn bộ).

• Class method có thể trả nil → chain bị gãy.
• Với logic phức tạp hoặc cần trả nil có chủ đích → class method.

A scope is a named query defined on the model that returns an ActiveRecord::Relation and is therefore chainable.

class Article < ApplicationRecord
  scope :published, -> { where(status: "published") }
  scope :recent,    -> { order(created_at: :desc).limit(10) }
  scope :by_tag,    ->(tag) { joins(:tags).where(tags: { name: tag }) }
end

Article.published.recent
Article.published.by_tag("ruby")

Scope vs class method: a scope always returns an ActiveRecord::Relation (even when the condition is false — it returns all).

• A class method can return nil, breaking a chain.
• Use class methods when logic is complex or when intentionally returning nil is desired.