Ruby on Rails - Accessing data in :has_many => :through association

Question

Hi, I'm trying to get to grips with this type of association in Rails

I have the following three models:

User

has_many: 'memberships'
has_many: groups, :through => 'memberships'

Membership

belongs_to: 'user'
belongs_to 'group'

Group

has_many: 'memberships'
has_many: 'users', :through => 'memberships'

I have a before_create method which adds a new user to a default user group.

user.rb

  attr_accessor :group_ids

  before_create :add_user_to_default_group
  after_save :update_groups

  def add_user_to_default_group
      self.group_ids = [1]
  end



 #after_save callback to handle group_ids
  def update_groups
    unless group_ids.nil?
      self.memberships.each do |m|
        m.destroy unless group_ids.include?(m.group_id.to_s)
        group_ids.delete(m.group_id.to_s)
      end 
      group_ids.each do |g|
        self.memberships.create(:group_id => g) unless g.blank?
      end
      reload
      self.group_ids = nil
    end
  end

I'm trying to write a test to ensure this is happening correctly.

user = User.new
user.username = 'testuser'
user.email = '[email protected]'
user.password = 'testpass'
user.password_confirmation = 'testpass'
assert user.save
assert user.errors.empty?
assert !user.memberships.empty?

The test passes if I use user.memberships.empty, but not if I use user.groups.empty?. I was under the impression I should be able to access the groups through .groups?

How would I test that the user belongs to a default group called 'Active Users'?

Any advice would be greatly appreciated.

Thanks