Embed scoped collections inside :conditions option as SQL

When scoped collections are used as a value for some attribute in :conditions clause, the generated SQL contains "attribute IN ( ... )" clause, where "( ... )" is enumeration of values of scoped collection. To do that an extra SQL clause is executed to get items of scoped collection. But in fact scoped collection can be inserted as a sub-SELECT.

Example:

@@@ ruby
    class PaymentType < AR::Base
      named_scope :credit_cards, :conditions => { :credit_card => true }
    end

    Orders.all(:conditions => { :payment_type_id => PaymentType.credit_cards })
@@@

This could be translated into a single query:

@@@ sql
    SELECT * from `orders` WHERE payment_type_id IN (SELECT id FROM `payment_types` WHERE credit_card = 1)
@@@

Solution:

Specially treat AR::NamedScope::Scope values in AR::Base#quote_bound_value

@@@ ruby
    def quote_bound_value(value) #:nodoc:
      if ActiveRecord::NamedScope::Scope === value
        scope = value.scope(:find) || {}
        field = (scope[:select] || value.primary_key).to_s.split(/,/).first.strip
        return value.construct_finder_sql(:select => field)
      end

      ...
@@@

This patch allows
1. Select #primary_key attribute by default.
2. If :select option is present, use first attribute specified.

If for some reason scoped collection should be embedded as enumeration, one can e.g. instantiate it manually (by calling #all or #to_a).