Single Resource incorrectly identified as a collection #245

Expected Behavior

A single Object passed into a serializer should use hash_for_one_record

Actual Behavior

A single Object of type Stripe::PaymentIntent is using hash_for_collection instead

Steps to Reproduce the Problem

1. get a single object of type Stripe::PaymentIntent, which responds to each, but not size or each_pair
2. try to serialize it - it will break with fast_jsonapi version >= 1.7.2
3. observe breakage, although resource responds to :id

Specifications

• Version: >= 1.7.2
• Ruby version: 3.2.2

Observations

> resource.class
=> Stripe::PaymentIntent
> resource.respond_to?(:id)
=> true
> resource.respond_to?(:size)
=> false
> resource.respond_to?(:each)
=> true
> resource.respond_to?(:each_pair)
=> false
> resource.id
=> "pi_3xxxxxxxxxxxxxxxxxxxxxxW"

Yes, it is odd that this Stripe object responds to :each for a single object, but shouldn't a response to :id be a stronger indicator that this is not a collection? 🤔

> x =  Stripe::PaymentIntent.new
=> #<Stripe::PaymentIntent:0x2feb8> JSON: {
}
> x.respond_to?(:each)
=> true

🤷

Root-Cause

This Change from April 11, 2020
from this PR
changed the behavior how #is_collection? determines if a resource is a collection.

Suggested Fix:

in is_collection? it might be more reliable to first check if the resource responds to :id. If so, it is a single resource, otherwise test for responds to :each, :each_pair, or :size

e.g.: instead of doing this:

    def is_collection?(resource, force_is_collection = nil)
      return force_is_collection unless force_is_collection.nil?

      resource.respond_to?(:each) && !resource.respond_to?(:each_pair)
    end

Could we do this?

    def is_collection?(resource, force_is_collection = nil)
      return force_is_collection unless force_is_collection.nil?
      
      # shouldn't this be a good indicator of a single record?
      return false if resource.respond_to?(:id)

      # otherwise let's guess
      resource.respond_to?(:each) && !resource.respond_to?(:each_pair)
    end