Field::Polymorphic accepts a call-able for the classes option

[brent-yearone]

This commit allows a field definition like this:

ATTRIBUTE_TYPES = {
  listable: Field::Polymorphic.with_options(classes: -> { Listable.listable_classes })
}

In my app I use a pattern where I have polymorphic associations that use a module to come into being. The module defines the associations and any special behaviors, and the model on the other side of the polymorphic relationship just includes the module to get the association and all the behaviors. At that time, the module registers the calling class. In the example above, the module is Listable; any model that includes Listable then has_many :lists, as: :listable.

It's a great pattern. The trouble is, the administrate dashboards get parsed before my models, so without the call-able classes option, Listable.listable_classes ends up empty all the time. By making it callable, it's evaluated at runtime, after the models are parsed, so we get the correct class list. Yay for dynamic discovery!

[pablobm]

Sounds good to me 👍 Thank you for the PR.

[brent-yearone]

You're most welcome! Thank you for all the work you do to maintain this most excellent library 😀