has_many_through with conditions on another table (:joins)


I have an class that looks like this:

class Section < ActiveRecord::Base   belongs_to :offering, :foreign_key => :parent_id   has_many :memberships, :as => :member_of   has_many :students, :through => :memberships, :source => :user,         :joins => "inner join roles onmemberships.role_id = roles.id", :conditions =>"roles.name = 'Student'" end

This doesn't work, though, because :joins isn't an allowed key. From associations.rb:

        def create_has_and_belongs_to_many_reflection(association_id, options, &extension)           options.assert_valid_keys(             :class_name, :table_name, :join_table, :foreign_key, :association_foreign_key,             :select, :conditions, :include, :order, :group, :limit, :offset,             :uniq,             :finder_sql, :delete_sql, :insert_sql,             :before_add, :after_add, :before_remove, :after_remove,             :extend, :readonly,             :validate           )

I added :joins to the list of allowed keys, and it made my tests pass, but I want to check that I'm not doing something silly. Is there a particular reason that :joins isn't an allowed key for has_many :through?

Correction: I made the change to :create_has_many_reflection

If there is no reason not to allow the :join keyword, what is the best way to make the change? I'd prefer not to freeze Rails, but I'm having a lot of trouble targeting just this method with a plugin.

I was able to get this to work by using :include => {:memberships => :role} but that's far from ideal as Ruby will build a lot of objects I don't need, and waste a significant amount of time and memory doing it.