In this case relations are not saved in both directions

class Site
  include Mongoid::Document
  field :name, :type => String

  has_one :siteHost, :autosave => true

end

class SiteHost
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name, :type => String

  belongs_to :site

end

if you try this example in the console
1. > site = Site.create(name: "ups", siteHost: SiteHost.create(name: "www.domain1.com"))
2. > sitehost = SiteHost.create(name: "www.domain2.com")

this one is fine:
3. > site.siteHost= sitehost
4. > site.save
5. > site.reload
6. > site.siteHost
you will get www.domain2.com

this one will fail:
3. > sitehost.site= site
4. > sitehost.save
5. > site.reload
6. > site.siteHost
you will still get www.domain1.com