gistfile1.rb

require 'mongoid'

Mongoid.configure { |config| config.master = Mongo::Connection.new.db 'mongoid_test' }

class User
  include Mongoid::Document

  field :email
  field :name

  validates_uniqueness_of :email
end

User.create :email => '[email protected]'

user = User.create # => #<User _id: 4cdee1080de0690f96000002, email: nil, name: nil>
user.update_attributes :email => '[email protected]' # false

user.errors       # => {:email=>["is already taken"]}
user.attributes   # => {"_id"=>BSON::ObjectId('4cdee1080de0690f96000002'), "email"=>"[email protected]"}
user.errors.clear # => {}
user.reload       # => #<User _id: 4cdee1080de0690f96000002, email: nil, name: nil>

user.update_attributes :name => 'Chris' # => true 
user.attributes # => {"_id"=>BSON::ObjectId('4cdee1080de0690f96000002'), "name"=>"Chris"}

# All as expected, until...
user.reload # => #<User _id: 4cdee1080de0690f96000002, email: "[email protected]", name: "Chris"> 
# Uh-oh!

User.where(:email => '[email protected]').count # => 2