Why Rails fails to detect AJAX request in production?

Question

I am using rails 5.2.4.1, and I am wondering why I get this error when I try to access the API endpoint:

ActionView::MissingTemplate (Missing template api/schools/classrooms, application/classrooms with {:locale=>[:en], :formats=>[:json], :variants=>[], :handlers=>[:raw, :erb, :html, :builder, :ruby, :haml]}.

here is my action:

def classrooms
    render json: {classrooms: user.daip.class_rooms.as_json(:include => [:users]), max_classrooms: user.daip.classrooms} , content_type: 'application/json'
  end

I tried also to add default json response to all classrooms_controller as:

resources :schools, :defaults => { :format => 'json' }

I tried to add .json to the route but also, did not work

how can I debug this? as it works locally, but not at the production server? I am using nginx with passenger.

What could be the issue?

EDIT

I also tried:

ActiveModel::Serializer.config.adapter = ActiveModel::Serializer::Adapter::JsonApi

EDIT2 I found that header HTTP_ACCEPT is passed as:

"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9"

I set it to application/json, text/plain, */* , still, rails search for a template!!

EDIT3

I tried to set default request to json as:

request.format = :json

and I tried to use format.json as:

def classrooms
    request.format = :json
    format.json {
    render plain: {classrooms: user.daip.class_rooms.as_json(:include => [:users]), max_classrooms: user.daip.classrooms}.to_json , content_type: 'application/json'
    }
  end

And I still have the same error, searching for a template..

EDIT4

Here is my request headers:

Host: myapp.com
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0
Accept: application/json, text/plain, */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Content-Type: application/json
Referer: https://myapp.com/en/api/school-admin
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
TE: Trailers
Origin: https://myapp.com
Content-Length: 0

and route:

namespace :api, defaults: {format: :json} do
    get 'classrooms' => 'schools#classrooms'
  end

Answer 1

After burning long previous hours of frustration, I decided to debug the code it self that I was trying to create json object of. I passed an empty json object, and then, I got status 200 response with empty json object! So, the error ActionView::MissingTemplate was completely misleading!!, made me blind of the real error. I ran the code user.daip.class_rooms.as_json(:include => [:users]) in rails console, and I got this error:

Cannot have a has_many :through association 'ClassRoom#users' which goes through 'ClassRoom#class_rooms_users' before the through association is defined

Checked class_room model, and oh! I just had to flip those lines:

  has_many :users, through: :class_rooms_users
  has_many :class_rooms_users

to:

  has_many :class_rooms_users 
  has_many :users, through: :class_rooms_users

That was really sad story, the motto is, never trust error message when every thing seems to be fine, sad developers.