has_many through question

Hi, I have a fairly typical has_many, through setup with a bit of a twist.

class Course < ActiveRecord::Base

  has_many :placements
  has_many :students, :through => :placements

end

class Student < ActiveRecord::Base

  has_many :placements
  has_many :courses, :through => :placements

end

class Placements < ActiveRecord::Base

  belongs_to :course
  belongs_to :student

end

So with this I could grab all the students for a course by:

@students = @course.students

Everything is fine so far, but here is the twist. I have to narrow things down by some data on the placements table. Let’s say I have a column called “placed_by” on “Placements”

How would I retrieve all of the students for a course if I only wanted to see those who were placed by “Bob” for example?

Thanks, #2 nails it! Would it be roughly the same if there were two or three unrelated fields on there? Something like:

has_many :students, :through => :placements do
def find_by(who, when)
find(:all, :conditions =>[‘placed_by = ?, created_on >= ?’,who, when])

end
end

@students = @course.students.find_by(who_id, datetime)

What about something a little different?
Is there a way to process 2 levels deep?
so @all_courses = @students.courses
In SQL that could be an IN() statement composed of student IDs.