Interview Question #2

question2.rb

# (Rails 3.2)

# Number of Units in the system: 30,000
# Number of Courses in the system: 50.

# Unit:
#   Properties
#   -- active: boolean
#   -- course_id: int

class Unit < ActiveRecord::Base
  attr_accessible :active
  belongs_to :course
end

# Course:
class Course < ActiveRecord::Base
  has_many :units
end

# This code finds all the ids of courses associated to *active* units.
Unit.where(active: true).all.map(&:course_id).uniq

# What is the main problem with the line above?