fix: format model polymorphic type from resource object type

[bf4]

In our app which is currently on the latest 0.9 we have a test
failing on the 0-11-dev branch (which was also failing on 0-10)

which posts the JSON body

{
  "data": {
    "type": "broker-memberships",
    "relationships": {
      "organization": {
        "data": {
          "type": "brokers",
          "id": "1"
        }
      },
      "user": {
        "data": {
          "type": "users",
          "id": "2"
        }
      }
    },
    "attributes": {
      "external-employee-id": "123"
    }
  },
}

to v1/broker-memberships

BrokerMembership is an STI ActiveRecord model
below Membership like below.

class Membership < ApplicationRecord
  belongs_to :organization, optional: false, polymorphic: true
  belongs_to :user, optional: false
end
class BrokerMembership < Membership
  validates :organization_type, inclusion: {in: ["Broker"], message: "must be Broker"}
end
class User < ApplicationRecord
end
class Broker < ApplicationRecord
end

our BrokerMembership resource looks like

module V1
  class MembershipResource < BaseResource

    has_one :organization,
      polymorphic: true,
      always_include_linkage_data: true,
      eager_load_on_include: false,
      create_implicit_polymorphic_type_relationships: false

    has_one :user,
      always_include_linkage_data: true
  end
end

In _replace_fields you can see the parsed field_data is

{
  :attributes=>{
    :external_employee_id=>"123"
  },
  :to_one=>{
    :organization=>{:id=>1, :type=>:brokers},
    :user=>2
  },
  :to_many=>{
  }
}

And in the call to _replace_polymorphic_to_one_link (as defined in
JSONAPI::ResourceCommon)

def _replace_polymorphic_to_one_link(relationship_type, key_value, key_type, _options)
  # relationship_type #=> "organization"
  relationship = self.class._relationship(relationship_type.to_sym)

  # self.organization_id = { type:  :brokers, id: 1 }
  send("#{relationship.foreign_key}=", {type: key_type, id: key_value})
  @save_needed = true

  :completed
end

calls the method defined by JSONAPI::ResourceCommon.define_on_resource like

def define_foreign_key_setter(relationship)
  if relationship.polymorphic?
    define_on_resource "#{relationship.foreign_key}=" do |v|
      _model.method("#{relationship.foreign_key}=").call(v[:id])
      _model.public_send("#{relationship.polymorphic_type}=", v[:type])
    end

which practially speaking calls

_model.organization_id = 1
_model.organization_type = :brokers

This latter behavior causes a failure since the 'organization_type' should be 'Broker' and not ':brokers'.

I'm not sure if this should be addressed in the definition of define_on_resource (by making it lookup the polymorphic type)
or in the JSONAPI::Request(Parser).parse_to_one_relationship (by removing the unformat and calling classify) or elsewhere.

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Bug fixes and Changes to Core Features:

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• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[bf4]

just for greppability

[bf4]

💯

[bf4]

I removed the instantiation of the method object here in favor of public send since we couldn't think of a reason the method object would be preferable

[bf4]

🔨 the fix