Assigning to has_many :through using the generated *_ids method does not insert in the same order

Suppose you have:

class Post
  has_many :readers
  has_many :people, :through => :readers
end

The following assignment will insert in the expected order:

@post.people = [@person1, @person2, @person3]

However this assignment will insert in an arbitrary order:

@post.person_ids = [@person1.id, @person2.id, @person3.id]

The fix for this looks a little ugly, but it performs well... we just create a hash of all the fetched records by id, then map the original ids to their respective objects (an O(n) operation in other words).

Attached is failing test case (and confirmation of the original correct behavior for the first case) plus fix. Tested against mysql, postgres and sqlite.