Possibly incorrect `after_initialize` usage

[zverok]

The case where it got me is somewhat convoluted (though real), yet I believe that it exposes some conceptual problem.

The case is this:

class Rate < ApplicationModel
  # columns are: rate_type, rate_value, rate_currency, etc.

  str_enum :rate_type, %w[fixed hourly], allow_nil: true, default: nil
  attr_readonly :rate_type
end

Since Rails 7.1, the default behavior of attr_readonly is to raise when #rate_type= is used on already persisted model. And that's where str_enum introduces a problem:

rate = Rate.create!(rate_type: nil, rate_value: 10)

Rate.find_by(rate.id) # ActiveRecord::ReadonlyAttributeError: rate_type

The culprit is here:

        after_initialize do
          send("#{column}=", default_value) if has_attribute?(column) && !try(column)
        end

What we have here is:

1. after_initialize is triggered after the model is loaded also
2. rate_type is nil (as loaded from DB), thus !try(:rate_type) is true
3. thus, str_enum tries to do self.rate_type = nil (which would be pointless anyway), triggering the "it tries to write to readonly column".

I believe the most comprehensive fix is to NOT set anything in after_initialize if it is an initialization of a model loaded from DB (i.e. if persisted?). As far as I understand, the intention of this block is that when the new record is initialized, the default should be set.

Or I might be missing something.