1

I'm trying to setup a polymorphic association using the jsonapi-resources gem in Rails 5.

I have a User model that has a polymorphic association called profile, which can be of type Inspector or Buyer. Here are the truncated models:

class User < ApplicationRecord
  belongs_to :profile, polymorphic: true
end

class Inspector < ApplicationRecord
  belongs_to :user
end

class Buyer < ApplicationRecord
  belongs_to :user
end

In the users table, there are corresponding profile_id and profile_type fields to represent the polymorphic association to inspectors and buyers. This all works as expected in our current Rails setup but I'm running into errors when trying to set this up for JSON:API using jsonapi-resources.

And now the corresponding jsonapi-resources resources and controllers (according to the directions):

class Api::V1::Mobile::UserResource < JSONAPI::Resource
  immutable
  attributes :name, :email
  has_one :profile, polymorphic: true
end

class Api::V1::Mobile::ProfileResource < JSONAPI::Resource
end

class Api::V1::Mobile::ProfilesController < Api::V1::Mobile::BaseController
end

As far as I can tell, everything should now be setup properly but I get the following error when hitting the endpoint:

"exception": "undefined method `collect' for nil:NilClass",
        "backtrace": [
          ".rvm/gems/ruby-2.6.5/gems/jsonapi-resources-0.10.2/lib/jsonapi/relationship.rb:77:in `resource_types'",

When digging into relationship.rb mentioned in the stack trace it looks like it can't get resolve the polymorphic types, so I tried the following:

class Api::V1::Mobile::UserResource < JSONAPI::Resource
  immutable
  attributes :name, :email
  has_one :profile, polymorphic: true, polymorphic_types: ['inspector', 'buyer']
end

But alas, another error: Can't join 'User' to association named 'inspector'; perhaps you misspelled it?

Thanks in advance for any help with getting this setup!

jdixon04
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1 Answers1

1

The core problem actually has nothing to do with jsonapi-resources and is the associations. The inverse side of a belongs_to resource is always a has_one or has_many which points to the foreign key on the other table (and vice-versa).

class User < ApplicationRecord
  belongs_to :profile, polymorphic: true
end

class Inspector < ApplicationRecord
  has_one :user, as: :profile
end

class Buyer < ApplicationRecord
  has_one :user, as: :profile
end

Having two belongs_to associations that point to each other would mean that you would have foreign keys on both sides - which is bad DB design due to the duplication (there should be only one source of truth) and won't really work in ActiveRecord since it will only ever write the foreign key on one side when you associate two models.

max
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  • Thanks, @max! I will give this change a shot. That said, is Rails just letting us get away with having two `belongs_to` associations even though it technically isn't correct? – jdixon04 Nov 06 '20 at 16:50
  • Rails doesn't actually look at the model on the inverse side until you actually use the association and fetch the records. This is really what you would expect in a dynamic language like Ruby where most things are resolved at runtime. – max Nov 06 '20 at 17:39
  • Just gave the suggested changes a shot but I still receive the same errors – jdixon04 Nov 06 '20 at 17:48
  • The docs are kind of fuzzy but I think you might need `relationship :profile, polymorphic: true` instead of `has_one :profile` in `UserResource`. I'm guessing that `relationship` corresponds to `belongs_to`. https://jsonapi-resources.com/v0.10/guide/resources.html#Relationships – max Nov 06 '20 at 17:57
  • No luck on that one :( This is the offending code in the library: https://github.com/cerebris/jsonapi-resources/blob/v0.10.2/lib/jsonapi/relationship.rb#L65. Noticed it was only looking at `has_many` so not sure if that has somehting to do with it – jdixon04 Nov 06 '20 at 18:02
  • I realize it's not a `has_many` but I tried changing it to ` has_many :profile, polymorphic: true` and it doesn't crash but when I try and include the `profile` I get "Cannot eagerly load the polymorphic association :profile" – jdixon04 Nov 06 '20 at 18:08