Narrowing an average to a specific

Simple question on using averages in Rails. I'm just trying to calculate and average for subset of bids in my table, those that apply to a single item.

Bid.average(:amount, :conditions => ['item_id = ?', 'item_id'])

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I can't quite figure out why this generates the following, with 'item_id', rather than the value of the item_id, which should be 5. . .

Parameters: {"id"=>"5"}
Item Load (0.1ms)  SELECT "items".* FROM "items" WHERE ("items"."id" = 5) LIMIT 1
Bid Load (0.5ms)  SELECT "bids".* FROM "bids" WHERE ("bids".item_id = 5) ORDER BY created_at DESC
Rendered bids/_bid.html.erb (6.6ms)
SQL (0.2ms)  SELECT AVG("bids"."amount") AS avg_id FROM "bids" WHERE (item_id = 'item_id')

Thanks!

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Because it's what you've told to do :) You're passing a string 'item_id' as the second element of the conditions array, and it's put into the place of the question mark. If you have a variable item_id that contains the id, you don't need to put it into quotemarks:

Bid.average(:amount, :conditions => ['item_id = ?', item_id])

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I think it should be this:

Bid.average(:amount, :conditions => ['item_id = ?', item_id])

item_id is getting put in there as a string, hence the quotes. Make sure to set item_id as a variable before such as:

item_id = params[:item_id] #or whatever the params are
Bid.average(:amount, :conditions => ['item_id = ?', item_id])