has_one SQL not using LIMIT when using include

[kevintraver]

I have the following model:

class Foo < ActiveRecord::Base
  has_many :messages
  has_one :last_message, -> { order(created_at: :desc) }, class_name: 'Message'
end

and following resource:

class FooResource < JSONAPI::Resource
  has_one :last_message, class_name: 'Message'
end

When I request /foo/1/last-message it generates the following SQL:

SELECT  `messages`.* FROM `messages` WHERE `messages`.`foo_id` = 1  ORDER BY `messages`.`created_at` DESC LIMIT 1

But when I request /foo/1?include=last-message, the SQL doesnt append LIMIT

SELECT `messages`.* FROM `messages` WHERE `messages`.`foo_id` IN (1)  ORDER BY `messages`.`created_at` DESC

[kevintraver]

rails/rails#10621

[kevintraver]

The issue is with this line, which always uses include.

jsonapi-resources/lib/jsonapi/resource.rb

Line 537 in ce94ced

records = records.includes(model_includes)

Perhaps it can check if the relationship is a has_one and use eager_load instead.

[tommy-russoniello]

Eager loading doesn't seem like a reliable solution as it won't combine with other queries well. I think the best we could really do until this is resolved on the Rails side (which it may never be) is to to get the id of the one related entity and specifically include it. This would slow down normal has_ones though so if we do that, it should be optional.