class Foo
  include Mongoid::Document
  field :name
  embeds_many :bars
  accepts_nested_attributes_for :bars, :allow_destroy => true
end

class Bar
  include Mongoid::Document
  field :name
  embedded_in :foo
end

foo = Foo.create(:name => "foo1", :bars => [{:name => "bar1"}, {:name => "bar2"}])
foo.reload

params = {
  "foo" => {
    "bars_attributes" => {
      "0" => {"name" => foo.bars[0].name, "id" => foo.bars[0].id, "_destroy" => "false"},
      "1" => {"name" => foo.bars[1].name, "id" => foo.bars[1].id, "_destroy" => "1"},
      "12341" => {"name" => "bar3", "_destroy" => "false"},
    }
  }
}

foo.safely.update_attributes(params["foo"])
# Mongo::OperationFailure: 10150: Field name duplication not allowed with modifiers
# from /home/rubish/.rvm/gems/ruby-1.9.2-p180/gems/mongo-1.3.1/lib/mongo/connection.rb:451:in `send_message_with_safe_check'

# operation sent to mongodb if not using safely
# MONGODB alma_connect['foos'].update({"_id"=>BSON::ObjectId('4e653346932ecf65a1000009')}, {"$pullAll"=>{"bars"=>[{"name"=>"bar2", "_id"=>BSON::ObjectId('4e653346932ecf65a100000b')}]}, "$pushAll"=>{"bars"=>[{"name"=>"bar3", "_id"=>BSON::ObjectId('4e653346932ecf65a100000c')}]}})

class Foo
  include Mongoid::Document  
  embeds_many :bars
  accepts_nested_attributes_for :bars, :allow_destroy => true
  field :name
end

class Bar
  include Mongoid::Document
  embedded_in :foo
  embeds_one :baz
  accepts_nested_attributes_for :baz, :allow_destroy => true
  field :name
end

class Baz
  include Mongoid::Document
  embedded_in :bar
  field :name
end

foo = Foo.create(:name => "foo1")

params = {
  "foo" => {
    "bars_attributes" => {
       "0" => {
         "name" => "bar1",
         "baz_attributes" => {"name" => "baz1"}
       }
    } 
  }
}

foo.safely.update_attributes(params["foo"])
# Mongo::OperationFailure: 10151: have conflicting mods in update
# from /home/rubish/.rvm/gems/ruby-1.9.2-p180/gems/mongo-1.3.1/lib/mongo/connection.rb:451:in `send_message_with_safe_check'

# operation sent to mongodb if not using safely
# MONGODB alma_connect['foos'].update({"_id"=>BSON::ObjectId('4e6532c2932ecf6591000009')}, {"$pushAll"=>{"bars"=>[{"name"=>"bar1", "_id"=>BSON::ObjectId('4e6532c4932ecf659100000a'), "baz"=>{"name"=>"baz1", "_id"=>BSON::ObjectId('4e6532c4932ecf659100000b')}}]}, "$set"=>{"bars.baz"=>{"name"=>"baz1", "_id"=>BSON::ObjectId('4e6532c4932ecf659100000b')}}})