Custom errors and valid property on Operations

[jwoertink]

Fixes #529

This allows you to set custom errors not related to attributes. When you set an error message, the whole operation is no longer valid.

The main use case is when your operations (or save operations) need some external object, but that object itself may also not be valid.

class SaveMessage < Message::SaveOperation
  needs user : User

  before_save do
    if user.is_not_allowed_to_save_message?
       add_error(:user_not_welcome, "You don messed up A A ron.")
    end
  end
end

SaveMessage.create(user: current_user) do |op, _|
  op.errors[:user_not_welcome] == ["You don messed up A A ron"]
end

[matthewmcgarvey]

@jwoertink Calling add_error does not make the operation invalid, is that intentional?

This is something that has tripped me up in rails before, and this PR implements the same thing.

[jwoertink]

hmm 🤔 I guess I assumed you'd never call add_error unless it was invalid... BUT.... you do make a good point. I think the ruby mutations gem (lol) does that where you just add an error and the whole thing fails. Do you think that would be a better way? So like, instead of setting self.valid = false, you just add an error message, and it knows it's invalid at that point.... I'm thinking maybe that is a better UX.

def valid?
  return false if any_errors_detected?

   #... other stuff it does
end

[matthewmcgarvey]

maybe the valid? check would just see if any errors are present?

[jwoertink]

Yup, exactly. Ok. I'll make that change and push it up.

[jwoertink]

Alright, so this doesn't really fix my original thought behind #529, but it does allow me to do some of what I intended by setting a custom error on operations.

I'm thinking maybe in a future PR we could have some way to "halt" the entire chain of callbacks and such. Something that says "don't run any more callbacks, don't run any other validations... nothing... just stop and exit the operation". For now, this will be a good improvement.