Array updating fails based on context?

class Asset
  include Mongoid::Document
  field :name
  field :peer_ids, :type => Array, :default => []

  def link_asset(asset_id)
    unless self.peer_ids.include?(asset_id) || self.id == asset_id
      self.peer_ids << asset_id
      Asset.find(asset_id).link_peer(self.id)
    end
  end

  def unlink_asset(asset_id)
    unless self.id == asset_id
      self.peer_ids.delete asset_id
      Asset.find(asset_id).unlink_peer(self.id)
    end
  end

  def link_peer(peer_id)
    unless self.peer_ids.include?(peer_id) || self.id == peer_id
      self.peer_ids << peer_id
      self.save
    end
  end

  def unlink_peer(peer_id)
    self.peer_ids.delete peer_id
    self.save
  end
end



ruby-1.9.2-rc2 > a1 = Asset.create(:name => 'Asset 1')
 => #<Asset _id: 4c4841d64889470bd3000001, name: "Asset 1", peer_ids: []> 
ruby-1.9.2-rc2 > a2 = Asset.create(:name => 'Asset 2')
 => #<Asset _id: 4c4841de4889470bd3000002, name: "Asset 2", peer_ids: []> 
ruby-1.9.2-rc2 > a1.link_asset(a2.id)
 => true 
ruby-1.9.2-rc2 > a1
 => #<Asset _id: 4c4841d64889470bd3000001, name: "Asset 1", peer_ids: [BSON::ObjectID('4c4841de4889470bd3000002')]> 
ruby-1.9.2-rc2 > a2
 => #<Asset _id: 4c4841de4889470bd3000002, name: "Asset 2", peer_ids: []> 
ruby-1.9.2-rc2 > a2.link_peer(a1.id)
 => true 
ruby-1.9.2-rc2 > a2
 => #<Asset _id: 4c4841de4889470bd3000002, name: "Asset 2", peer_ids: [BSON::ObjectID('4c4841d64889470bd3000001')]> 
ruby-1.9.2-rc2 >