Check joins when determining preloading of includes

Currently when calling ActiveRecord::Base#find with both an include clause and an order clause which references the include, ActiveRecord::Associations::ClassMethods#find_with_associations is used. This discards the select clause. In some cases, this is not the intended behavior. Take the following contrived example:

Person.find(:all, :select => "people.*, email.address IS NOT NULL AS has_email", :order => "email.address", :include => :email, :phone, :address)

The generated query will end up looking something like:

SELECT `people`.`id` AS t0_r0, `people`.`first_name` AS t0_r1 ... FROM `people` LEFT OUTER JOIN `emails` ON `people`.id = `emails`.person_id LEFT OUTER JOIN `phones` ON `people`.id = `phones`.person_id LEFT OUTER JOIN `addresses` ON `people`.id = `addresses`.person_id ORDER BY email.address;

As you can see, the provided select clause has been discarded in place of the complex one needed to load the includes.

I suggest that the user should be able to also specify a join clause which, if it includes the right tables, will allow ActiveRecord::AssociationPreload::ClassMethods#preload_associations to be used instead.

This has a two fold benefit of allowing a select clause to be used. And of still splitting the eager loading into multiple queries.

A revised query would look something like:

Person.find(:all, :select => "people.*, email.address IS NOT NULL AS has_email", :order => "email.address", :joins => :email, :include => :email, :phone, :address)

With the first query (there will now be multiple for eager loading), looking like:

SELECT people.*, email.address IS NOT NULL AS has_email FROM `people` INNER JOIN `emails` ON `people`.id = `emails`.person_id ORDER BY email.address;

A patch for this functionality is attached.