ruby
class Account
  include Mongoid::Document
  
  field :following_ids, type: Array, default: []
  field :follower_ids,  type: Array, default: []
  
  has_and_belongs_to_many :followers, :class_name => 'Account', :foreign_key => 'follower_ids'
  has_and_belongs_to_many :followees, :class_name => 'Account', :foreign_key => 'following_ids'
end

 ruby
require 'test_helper'

class AccountTest < ActiveSupport::TestCase
  setup do
    @account = FactoryGirl.create(:account)
    @other_account = FactoryGirl.create(:account)
  end

  test "if count works" do
    assert_difference "@account.followees.count" do
      @account.followees << @other_account
    end
  end

  test "if same thing with length works" do
    assert_difference "@account.followees.length" do
      @account.followees << @other_account
    end
  end
end

AccountTest:
     FAIL if count works (0.01s) 
          "@account.followees.count" didn't change by 1.
<1> expected but was
<0>.
          /Users/marc/.rvm/gems/ruby-1.9.2-p180/bundler/gems/rails-e59491355e92/activesupport/lib/active_support/testing/assertions.rb:47:in `block in assert_difference'
     PASS if same thing with length works (0.00s)