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

f = Foo.create
Foo.collection.find({:_id => f.id}).update(:$set => {:old_property => "value"})
f.reload()
f.dup

Mongoid::Errors::UnknownAttribute:
Problem:
  Attempted to set a value for 'old_property' which is not allowed on the model Foo.
Summary:
  Without including Mongoid::Attributes::Dynamic in your model and the attribute does not already exist in the attributes hash, attempting to call Foo#old_property= for it is not allowed. This is also triggered by passing the attribute to any method that accepts an attributes hash, and is raised instead of getting a NoMethodError.
Resolution:
  You can include Mongoid::Attributes::Dynamic if you expect to be writing values for undefined fields often.
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/attributes/processing.rb:100:in `process_attribute'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/attributes/processing.rb:25:in `block in process_attributes'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/attributes/processing.rb:23:in `each_pair'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/attributes/processing.rb:23:in `process_attributes'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/document.rb:110:in `block in initialize'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/threaded/lifecycle.rb:84:in `_building'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/document.rb:104:in `initialize'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/copyable.rb:25:in `new'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/bundler/gems/mongoid-4294ce79f6aa/lib/mongoid/copyable.rb:25:in `clone'
	from (irb):23
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/gems/railties-4.2.0/lib/rails/commands/console.rb:110:in `start'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/gems/railties-4.2.0/lib/rails/commands/console.rb:9:in `start'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/gems/railties-4.2.0/lib/rails/commands/commands_tasks.rb:68:in `console'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/gems/railties-4.2.0/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
	from /Users/jonathan/.rvm/gems/ruby-2.1.5/gems/railties-4.2.0/lib/rails/commands.rb:17:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'2.1.5 :024 >