Our usage of ActiveModel::MissingAttribute is quite different than ActiveRecord's usage/intention.

Mongoid raises ActiveModel::MissingAttribute when using the .only projection method, like so:

Band.only(:name).first.label

However, Rails raises this error when attempting to call a getter which is not defined. From the Rails docs:

  # Raised when an attribute is not defined.
  #
  #   class User < ActiveRecord::Base
  #     has_many :pets
  #   end
  #
  #   user = User.first
  #   user.pets.select(:id).first.user_id
  #   # => ActiveModel::MissingAttributeError: missing attribute: user_id

In this same case, Mongoid would simply raise a plain-old MethodMissing error.

I propose to rename this error to Mongoid::Errors::AttributeNotLoaded to better clarify its intention. In addition, we should:

• Have this new error class inherit from Mongoid::Error::MongoidError, same as every other error in Mongoid.
• Address MONGOID-5320 (make error message more informative)