mongoid relational form example

### Models ###
class Post 
  include Mongoid::Document
  references_many :properties, :stored_as => :array, :inverse_of => :posts, :index => true
end

class Property 
  include Mongoid::Document
  field :name
  references_many :posts, :stored_as => :array, :inverse_of => :properties
end

### posts/_form.html.haml ###
- form_for @post do |f| 
    = select_tag 'post[property_ids][]', options_from_collection_for_select(Property.find(:all), :id, :name, @post.properties.map {|p| p.id}), {:size => 3, :multiple => true}
  %p
    = f.submit

### Post Controller ###
def create
  @post = Post.find(params[:id])
  if @post.save
    ...

########
# This sumbits a post that looks like this
########
Started POST "/posts" for 127.0.0.1 at Thu Jul 29 08:49:07 -0700 2010
  Processing by PostsController#create as HTML
  Parameters: {"commit"=>"Create Post", "post"=>{"property_ids"=>["4c5078608035442eb6000003"]}, "authenticity_token"=>"m44qVAUkzc7d0p1OT5JzlLJVtwdLeLmdIqB6nTkFjFQ=", "_snowman"=>"☃"}
  MONGODB dev['properties'].find({:_id=>{"$in"=>["4c5078608035442eb6000003"]}}, {})

########
# and then if we look at what was created we can see that the post was saved 
# with a property_ids array of strings instead of objectids and the 
# properties association doesn't work
########
$ rails c

>> post = Post.find(:last)
=> <Post _id: 4c51a2f48035442f7400000d, property_ids: ["4c5078608035442eb6000003"], user_id: BSON::ObjectID('4c5078608035442eb6000001')>

>> post.properties.to_json
=> "[]"