Move invitation invalidation to service

spec/services/create_account_spec.rb

@@ -23,6 +23,28 @@ RSpec.describe CreateAccount, type: :model do
     end
   end
 
+  describe "#update_invitation" do
+    let(:invitation) { create :invitation }
+    let(:service) { CreateAccount.new(
+      username: 'isaacnewton',
+      email: 'isaacnewton@example.com',
+      password: 'bright-ideas-in-autumn',
+      invitation: invitation
+    )}
+
+    before(:each) do
+      service.send(:update_invitation, 23)
+    end
+
+    it "marks the invitation as used" do
+      expect(invitation.used_at).not_to be_nil
+    end
+
+    it "saves the invited user's ID" do
+      expect(invitation.invited_user_id).to eq(23)
+    end
+  end
+
   describe "#add_ldap_document" do
     let(:service) { CreateAccount.new(
       username: 'halfinney',