Rails ActiveRecord Model not updating in RSpec model spec

This is driving me insane. I've stripped this down to the bare minimum without losing context (I think!)

All I am trying to do is check that when I update a value and save it to the database, that the value was saved. I want to do this because I need to write some other code that conditionally prevents this in the before_save callback, and I can't test that until I'm sure this is working!

The factories and the spec are below, I'm sure its something really stupid but I just can't figure it out...

FactoryGirl.define do

  factory :programme do
    name 'Trainee Programme'
  end

  factory :membership do
    programme
  end

  factory :specialty do
    sequence(:name) { |n| "Specialty #{n}" }
  end

  factory :user do
    sequence(:email) { |n| "factorygirl-user-#{n}@remailer.org" }
    password 'password'
    password_confirmation 'password'
    factory :trainee, class: User do
      sequence(:email) { |n| "factorygirl-trainee-#{n}@remailer.org" }
      name 'Factory Girl Trainee'
      after(:create) do |user|
        FactoryGirl.create(:membership, user: user, start_date: 1.day.ago)
      end
    end
  end

end

describe Membership do

  let(:trainee) { FactoryGirl.create(:trainee) }

  it 'sets specialty' do
    puts trainee.current_membership.inspect
    trainee.current_membership.specialty = specialty
    puts trainee.current_membership.inspect
    trainee.current_membership.save!
    puts trainee.current_membership.inspect
    expect(trainee.current_membership.specialty).to eq(specialty)      
  end

end

The spec is failing because the expect sees a nil value. When I run the code the debug output I get is:

#<Membership id: 11, user_id: 11, programme_id: 11, start_date: "2015-03-10", end_date: nil, created_at: "2015-03-11 22:02:51", updated_at: "2015-03-11 22:02:51", options: {}, specialty_id: nil, membership_type_id: nil>
#<Membership id: 11, user_id: 11, programme_id: 11, start_date: "2015-03-10", end_date: nil, created_at: "2015-03-11 22:02:51", updated_at: "2015-03-11 22:02:51", options: {}, specialty_id: nil, membership_type_id: nil>
#<Membership id: 11, user_id: 11, programme_id: 11, start_date: "2015-03-10", end_date: nil, created_at: "2015-03-11 22:02:51", updated_at: "2015-03-11 22:02:51", options: {}, specialty_id: nil, membership_type_id: nil>

So its as if the assignment of specialty never happens??