my models

class Auction
  belongs_to :item
  belongs_to :user, :foreign_key => :current_winner_id
  has_many :auction_bids

end

class User
  has_many :auction_bids

end

class AuctionBid
  belongs_to :user

end

current usage

An item is displayed on the page, the user enters an amount and clicks bid. Controller code might look something like this:

class MyController
  def bid
    @ab = AuctionBid.new(params[:auction_bid])
    @ab.user = current_user
    if @ab.save
      render :json => {:response => 'YAY!'}
    else
      render :json => {:response => 'FAIL!'}
    end
  end 
end

desired functionality

This works great so far! However, I need to ensure a couple other things happen.

1. @ab.auction.bid_count needs to be incremented by one.
2. @ab.user.bid_count needs to be incremented by one
3. @ab.auction.current_winner_id needs to be set to @ab.user_id

That is, the User and the Auction associated with the AuctionBid need values updated as well in order for the AuctionBid#save to return true.