I often modify relations by setting the underlying relation_id field. I have to do this all over the place to work with forms in Rails. We're usually talking about hidden form fields that contain the id, populated via autocomplete, drag and drop, etc. Unfortunately setting the relation_id field doesn't flush the old relation proxy.

Test:

require "rubygems"
require "mongoid"

Mongoid::Config.from_hash({"database" => "mtest"})

class Library
  include Mongoid::Document
  field :name
end
class Book
  include Mongoid::Document  
  belongs_to :library
end

one, two = Library.create(:name => "one"), Library.create(:name => "two")

book = Book.new(:library => one)
p book.library.name
book.library_id = two.id
p book.library.name

Expected:
"one"
"two"

Actual:
"one"
"one"

The workaround for this problem involves strategically calling reload_relations, like so:

book.attributes = params[:book]
book.reload_relations

Unfortunately that will also flush out any embedded relations that might've gotten populated by attrbitutes=, so sometimes I have to do something more strenuous:

book.attributes = params[:book]
book.attributes.each_pair do |key, value|
  if key =~ /^(.*)_id$/
    relation = $1
    if book.instance_variable_defined?("@#{relation}")          
      book.remove_instance_variable("@#{relation}")
    end
  end
end

This is especially difficult to deal with inside callbacks. I basically have to defensively clear out relations all over the place.