Problems with Mongoid::Errors::Callback:

• Mongoid::Errors::Callback is not correctly worded. It says "...resulted in a false return from a callback", but as of Rails 5, returning false from an ActiveModel callback doesn't not have any effect: https://www.bigbinary.com/blog/rails-5-does-not-halt-callback-chain-when-false-is-returned
  • This wording should be changed to "...threw an :abort"
• In addition, the error does not tell us which callback actually failed.
• Lastly, in some cases, I think it is actually possible that this error is raised even if no callback was aborted, but if the save failed for any other reason, which is confusing. I see this happening in my app after upgrading to Mongoid 9, but I am still trying to pin down the actual problem.

To fix/clarify things, I'd like would propose the following enhancement:

1. Make a new error class "Mongoid::Errors::CallbackAborted". This should be raised during the actual abort, and make it clear in the error exactly which callback was aborted.
2. If no callback aborted, instead, raise a new generic error class "Mongoid::Errors::PersistenceFailed"
3. To preserve the legacy error, make:
   1. Mongoid::Errors::Callback = Mongoid::Errors::PersistenceFailed
   2. Mongoid::Errors::CallbackAborted < Mongoid::Errors::PersistenceFailed (CallbackAborted is a subclass)

—

Demonstration that returning false in a callback has no effect in MongoDB:

class Person
  include Mongoid::Document

  before_save :return_false

  def return_false
    throw(:abort)
  end
end

Person.new.save! #=> true, it works!

Instead, this error is raised if you throw(:abort)

class Person
include Mongoid::Document 
  before_save :throw_abort

  def throw_abort
    throw(:abort)
  end
end

Person.new.save! #=> raises error blelow

Mongoid::Errors::Callback: 
message:
  Calling save! on Person resulted in a false return from a callback.
summary:
  If a before callback returns false when using Document.create!, Document#save!, or Document#update_attributes! this error will get raised since the document did not actually get saved.
resolution:
  Double check all before callbacks to make sure they are not unintentionally returning false.