Ruby Operator Lookup

The ||= operator is a conditional assignment operator. It assigns the right-hand value to the left-hand variable if the left-hand value is falsy. It is often referred to as the memoization operator.

This is one of many operators where = gets tacked on to the end as a shorthand. These two code snippets are equivalent.

Using boolean or followed by an assignment:

who = nil # not set yet
who = who || 'friend'
puts "Hello, #{who}!" #=> Hello, friend!

Using the ||= operator:

who = nil # not set yet
who ||= 'friend'
puts "Hello, #{who}!" #=> Hello, friend!

Here is a more practical example.

class SomeController < ApplicationController
  def index
    @recent_books = books_by_status(:recent)
    @abandoned_books = books_by_status(:abandoned)
  end

  private

  def external_books
    @external_books ||= begin
      user_identifier = current_user.book_api_identifier

      ExternalBookApi.fetch_books(user_identifier) #=> array of book hashes
    end
  end

  def books_by_status(status)
    external_books.select { |book| book[:status] == status }
  end
end

We don't want to make an expensive, redundant, and possibly billable external API call multiple times if we don't have to. The ||= operator memoizes the result of that block in @external_books. Each subsequent call the external_books method will return the memoized value.

Or Equals Operator

References