Adding elementos into a collection

So I have this

  has_many :assistance
  has_many :users, :through => :assistance

  has_many :assistance
  has_many :assisted_events, :through => :assistance, :source =>

  belongs_to :user
  belongs_to :event

Users assisting to events are assigned using

@event.users << user
BUT assistance model also have an status attribute, so I need to fill it
at the same time of doing @event.users << user, is it possible to make
it on the same line? I actually dont know how to assign that attribute

Look for "Association callbacks"