Testing a Rails belongs_to model with Rspec

I have a Profile model:

class Profile < ActiveRecord::Base
  belongs_to :user

  def fields_needed_for_completion
    [self.name, self.city]
  end

  def completed?
    !fields_needed_for_completion.any? { |f| f.nil? || f == "" }
  end
end

I am trying to determine how to write unit tests for a model with a belongs_to association. In particular I'm not sure how to properly setup data for the tests.

So far, I've put together the following:

describe Profile do

  subject(:profile) { FactoryGirl.create(:profile) }

  describe "fields_needed_for_completion" do
    context "with all fields missing" do
      it "returns all fields as nil" do
        expect(profile.fields_needed_for_completion.all? &:blank?).to be true
      end
    end
  end

  describe "#completed?" do
    #TO DO
  end
end

Two questions:

1. Is it fine to use FactoryGirl to create the Profile object instead of calling Profile.create directly? There are no attributes set in the factory right now (i.e., the factory is defined like so: factory :profile do; end)

2. As you can see, the User model is not used at all in these specs. Is it appropriate to unit test a model in isolation like this, even though in practice it will belong to a user? Or should I somehow mock out the User?