Adding elementos into a collection

So I have this

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

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

Assistance   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"