Using a model for an association results in no PK error

My application has the concept of Users, Events, and Invitations. A user can be invited to many events, and an event can have many invited users; this relationship has a status attached to to it, which indicates whether the user has accepted the invitation or not.

I'm modeling the invitation by using an Invitation model for the join table between events and users, but I unable to actually update the join table through the Invitation model assigned to it:

class Event < Sequel::Model
  many_to_many :users, :join_table => :invitations
  one_to_many :invitations
end

class Invitation < Sequel::Model
  many_to_one :user
  many_to_one :event
end

class User < Sequel::Model
  many_to_many :events, :join_table => :invitations
  one_to_many :invitations

  def invite (user, event)
    user.add_invitation(event: event)
  end

  def decline(event)
    i = Invitation.where(user: self, event: event).first
    i.update(status: "declined")
  end
end

user_one = User.create(name: "foo")
user_two = User.create(name: "bar")
event = Event.create(title: "fete")

user_one.add_event(event)
user_one.invite(user_two, event)

user_two.invitations
# [#<Invitation @values={:id=>nil, :status=>"pending", :event_id=>1, :user_id=>2}>]
# id is nil, which probably leads to the error below?

user_two.decline(event)
# `pk': No primary key is associated with this model (Sequel::Error)`