Rails - Oct 2011 - Is this functionality achievable through accepts_nested_attributes_for

I have these four models

class Comment < ActiveRecord::Base
  has_many :relationships
  has_many :advantages, :through => :relationships, :source
=> :resource, :source_type => ''Advantage''
  has_many :disadvantages, :through => :relationships, :source
=> :resource, :source_type => ''Disadvantage''

  accepts_nested_attributes_for :advantages, :disadvantages
end

class Advantage < ActiveRecord::Base
  has_many :relationships, :as => :resource
  has_many :comments, :through => :relationships
end

class Disadvantage < ActiveRecord::Base
  has_many :relationships, :as => :resource
  has_many :comments, :through => :relationships
end

class Relationship < ActiveRecord::Base
  belongs_to :resource, :polymorphic => true
  belongs_to :comment
end


Since I have related commennts with advantages, disadvantages and have
given the attribute `accepts_nested_attributes_for`, when I try to
save new record of comments with new records of advantages, it saves.
For example

params = {
	:comment => {
	  :comment => ''This is a sample comment'',
	  :advantages_attributes => [
		{ :plus => ''This is a good product'' },
		{ :plus => ''It has all the functionality'' },
		{ :plus => ''Performs bette than others''}
	  ]
	}
}

comment = Comment.create(params[:comment])

This saves 3 records of advantages and 1 comment to the database and
makes 3 entries in the relationship table.

But not always I want to create new entries for the advantages, if
there are already existing advantages, I would like to send just the
id, so only the relatioship table is being updated. So my params would
be like this

params = {
	:comment => {
	  :comment => ''This is a sample comment'',
	  :advantages_attributes => [
		{ :id => 1 },
		{ :plus => ''It has all the functionality'' },
		{ :plus => ''Performs bette than others''}
	  ]
	}
}

Here I want a new comment to be created which has to be associated
with the already existing advantage with `id => 1` and create new
advantages for the next two and relate it with the comment, but when i
try this directly it throws an error. How can I achieve this?

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