Mongoid 3.1.6 update_attributes gotcha

console_3.1.6.log

[1] pry(main)> p=Post.create
=> #<Post _id: 53db80397526a46c23000001, >
[2] pry(main)>  p.title="foo"
NoMethodError: undefined method `title=' for #<Post _id: 53db80397526a46c23000001, >
from /home/koronen/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/mongoid-3.1.6/lib/mongoid/attributes.rb:320:in `method_missing'
[3] pry(main)>  p.update_attributes(title: 'a')
=> true
[4] pry(main)>  Post.first
=> #<Post _id: 53db80397526a46c23000001, title: "a">
[5] pry(main)>  _.title
=> "a"

console_4.0.0.log

irb(main):001:0> p=Post.create
=> #<Post _id: 53db86927061723c9b000000, >
irb(main):002:0> p.update_attributes(title: 'foo')
Mongoid::Errors::UnknownAttribute: 
Problem:
  Attempted to set a value for 'title' which is not allowed on the model Post.
Summary:
  Without including Mongoid::Attributes::Dynamic in your model and the attribute does not already exist in the attributes hash, attempting to call Post#title= 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.

Gemfile

gem 'mongoid', '3.1.6'

post.rb

# app/models/post.rb
class Post
  include Mongoid::Document
end