# Given a set of class names, will require objects of those types to have been populated
# Usage: depends_on(User, Post, Comment)
def depends_on(*args)
  raise "Depends on #{args.map{ |a| a.name.pluralize }.to_sentence.downcase} to populate data." if args.any? { |a| a.count.zero? }
  args.each { |a| eval "@#{a.name.underscore.pluralize} = #{a}.all" }
end