jsonapi_resource should use relationship.class_name instead of relationship_name

We have models Contractor and Mm::Kudo and corresponding resources.

class ContractorResource < JSONAPI::Resource
  model_name 'Coworker'
  attributes :name, :uid, :avatar_url

  has_many :kudos_given, class_name: 'Mm::Kudo'
end

class Mm::KudoResource < JSONAPI::Resource
  model_name 'Mm::Kudo'
  attributes :value, :tags, :comment

  has_one :giver, class_name: 'Contractor'
end

And generate route for kudos-received relation points to kudos#get_related_resources controller instead of mm/kudos#get_related_resources (btw.: includes work fine)

There two options:

1. leave the default behaviour as it is and allow to overwrite controller at relationship level

e.g.: for the given example

  has_many :kudos_given, class_name: 'Mm::Kudo', controller: 'mm/kudos'

and pass the relationship params to the jsonapi_related_resources

2. use model_name to generate path for default controller (and still allow to overwrite controller at the has_many definition

Which option should we pick and adopt

Start with the relationship definitions in the issue and trace how the kudos-received related-resource route selects its controller. Compare the relationship name with class_name and model_name, then decide whether default routing should use the model name or support a relationship-level controller override; done means the generated route targets the intended mm/kudos controller without breaking existing behavior.