has_many :through with instance specific conditions

Is it possible to set an instance-level constraint on a has_many, :through relationship in rails 3.1?

http://guides.rubyonrails.org/association_basics.html#the-has_many-association

Something like:

Class A
    has_many :c, :through => :b, :conditions => { "'c'.something_id" => @a.so开发者_如何学Cmething_id }

The documentation gives me hope with this, but it doesn't work for me:

If you need to evaluate conditions dynamically at runtime, you could use string interpolation in single quotes:

class Customer < ActiveRecord::Base
  has_many :latest_orders, :class_name => "Order",
    :conditions => 'orders.created_at > #{10.hours.ago.to_s(:db).inspect}'
end

That gives me "unrecognized token '#'" on rails 3.1. Wondering if this functionality doesn't work anymore?

EDIT

Want to clarify why I don't think scopes are the solution. I want to be able to get from an instance of A all of the Cs that have a condition (which is based on an attribute of that instance of A). These are the only Cs that should EVER be associated with that A. To do this with scopes, I would put a scope on C that takes an argument, and then have to call it from @a with some value? I don't get why that's better than incorporating it into my has_many query directly.

Use a scope on the orders model:

class Order < ActiveRecord::Base
  belongs_to :customer
  scope :latest, lambda { where('created_at > ?', 10.hours.ago) }
end

And then call it with:

@customer.orders.latest

And if you really want to use latest_orders, you can instead add this to the Customer model:

def latest_orders
  orders.where('created_at > ?', 10.hours.ago)
end