class Band
  include Mongoid::Document
  field :name, type: String
  field :origin, type: String

  attr_readonly :name, :origin
end

# These fail as documented
rails console> band = Band.first
rails console> band.update_attribute(:name, "Tool")
Mongoid::Errors::ReadonlyAttribute: 
Problem:
  Attempted to set the readonly attribute 'name' with the value: Tool.
Summary:
  Attributes flagged as readonly via Model.attr_readonly can only have values set when the document is a new record.
Resolution:
  Don't define 'name' as readonly, or do not attempt to update its value after the document is persisted.
	from /Users/joel/.rvm/gems/ruby-2.2.2/gems/mongoid-4.0.2/lib/mongoid/persistable/updatable.rb:29:in `update_attribute'
...

rails console> band.remove_attribute(:name)
Mongoid::Errors::ReadonlyAttribute: 
Problem:
  Attempted to set the readonly attribute 'name' with the value: nil.
Summary:
  Attributes flagged as readonly via Model.attr_readonly can only have values set when the document is a new record.
Resolution:
  Don't define 'name' as readonly, or do not attempt to update its value after the document is persisted.
	from /Users/joel/.rvm/gems/ruby-2.2.2/gems/mongoid-4.0.2/lib/mongoid/attributes.rb:144:in `remove_attribute'

# This does not fail, but I would expect it to
rails console> band = Band.first
rails console> band.name = 'new name'
rails console> band.save
=> true

# I would also expect this to fail
rails console> band = Band.first
rails console> band.name = 'new name'
rails console> band.save!
=> true