I think I'm hitting an edge case (again), this time around polymorphic relations. I've got this model, and I want to display the

recipients

in a list:

rb
class Mailing < ApplicationRecord
  belongs_to :mailable, polymorphic: true

  delegate :recipients, to: :mailable
end

and an example of a `mailable`:

rb
class Event < ApplicationRecord
  has_many :recipients
  has_many :mailings, as: :mailable
end

The

delegate

is necessary, because you can't specify a

has_many through:

on a polymorphic association:

rb
  has_many :recipients, through: :mailable

  # => Cannot have a has_many :through association 'Mailing#recipients' which goes through the polymorphic association 'Mailing#mailable'. (ActiveRecord::HasManyThroughAssociationPolymorphicThroughError)

I tried to solve this with an

Array

field, but failed because it needs an

ArrayResource

to back it. Is there any other way to achieve this?

Hi @millions-exabyte-27721 👋 The array field needs the array resources to function, you can easly create one recipt array resource

yep I got that, but I already have a

recipient

resource, i.e. the

User

resource. but that doesn't work because it needs an Array resource:

rb
    field :recipients, as: :array, use_resource: "User"

You must create an array resource, since we're not namespacing the resources types you need to use another name

yuck

True, the used resource is what diferentiates it from a has many

ugh, working with polymorphic assiciations is always such a brainfuck

What about:

has_many :recipients, -> (mailing) { mailing.recipients }

if I add a dummy

has_many

it actually works:

rb
  # this is a dummy association to appease Avo
  has_many :recipients, class_name: "User"
  delegate :recipients, to: :mailable

rb
    field :recipients, as: :has_many, use_resource: "User", name: "Recipients"

ruby
class Mailing < ApplicationRecord
  belongs_to :mailable, polymorphic: true

  has_many :recipients, -> (mailing) { mailing.recipients }
end

I dont have the context setted up but probbaly this works ☝️

final question though,

has_many

fields cannot be set to

readonly

, right?

actually i think it misses a step:

has_many :recipients, -> (mailing) { mailing.mailable.recipients }

right! that was what I tried

ActiveRecord::StatementInvalid (PG::UndefinedColumn: ERROR:  column users.mailing_id does not exist

don't worry though

readonly marks fields to not be included or updated on form submit

the

delegate

more or less is just overriding the getter, which is good in my case because I don't even want that list to be editable, I just want to display it for reference

No, actions are controller by policy

ah!

are you using authorization yet?

yep

Yes, that's intentional, it avoids letting actions authorized without explicitly authorize those

good. I just think I'll punt on it in this PR and do it later

Cool!

again, maybe something to write a guide for 😄

You mean the authorization?

well, the whole polymorphic shenanigans

Got it, makes sense. I'll add an issue for that so we don't lose track of it. Meanwhile, if you're up for contributing a guide about this issue, we'd love to merge it.