Topic: adding items to an event and increasing number

I have an app that have event, event cand have items, user can adquire items from event. but i dont know if this is correct.

-MODEL-

Event
has_many :user_items
has_many :items, :through => :user_items

user
has_many :user_items
has_many :items, :through => :user_items


Item
belongs_to :user_items

-CONTROLLER-

UserItems

  def add_item # This will add one item to an event for the user to adquire them
    event = Event.find(params[:id])
    event.items << Item.new(:name => params[:name], :quantity => params[:quantity])
  end

  def increase_item # This will assign a quantity of items for the user from the event items
    item = Item.find(params[:id])
    item.quantity -= params[:quantity]
    if item.save
      flash[:notice] = "Item Saved"
    else
      flash[:alert] = "Error"
    end
  end

Is this correct or is there a better way of doing it?