Rails rendering JSON causing helper method to be called twice - ruby-on-rails

Question

I'm getting strange behavior when I render JSON from my Rails app. A helper method is run twice when render :json is called. Here's the controller and method:
class UsersController < ApplicationController
def current
render json: { :errors => "Incorrect credentials" }, :status => :bad_request
end
end
I have the following helper module, with a puts statement for debugging:
module SessionsHelper
def current_user
puts "current_user"
if encrypted_id = request.headers[:user_id]
user = User.find_by(id: EncryptionService.decrypt(encrypted_id))
if user && user.authenticated?(request.headers[:remember_token])
#curent_user = user
end
end
end
end
The SessionsHelper is included in the Application Controller
class ApplicationController < ActionController::API
include SessionsHelper
end
When sent the request, I get the following:
Started GET "/user/current" for ::1 at 2021-02-12 22:06:47 -0800
Processing by UsersController#current as */*
Parameters: {"user"=>{}}
current_user
[active_model_serializers] Rendered ActiveModel::Serializer::Null with Hash (0.06ms)
Completed 400 Bad Request in 1ms (Views: 0.7ms | ActiveRecord: 6.6ms | Allocations: 383)
current_user was printed, even though the function was never called. When I comment out the render json: statement, leaving:
class UsersController < ApplicationController
def current
end
end
I get the following:
Started GET "/user/current" for ::1 at 2021-02-12 22:09:43 -0800
Processing by UsersController#current as */*
Parameters: {"user"=>{}}
Completed 204 No Content in 0ms (ActiveRecord: 4.2ms | Allocations: 78)
current_user is not printed. Why would render json: call current_user? In my actual application, this is causing the database to be hit twice (although Rails wisely caches the result).
UPDATE: I'm onto something here. I ran puts caller[0] to see who was calling the function. The result:
/Users/izaguirrejoe/.rbenv/versions/3.0.0/lib/ruby/gems/3.0.0/gems/active_model_serializers-0.10.12/lib/action_controller/serialization.rb:40:in 'serialization_scope'
def serialization_scope
return unless _serialization_scope && respond_to?(_serialization_scope, true)
send(_serialization_scope)
end
Any ideas?

Answer 1

I see that you are using active_model_serializers, if you check out their docs it says here, the default serialisation scope is :current_user. It also emphasizes that
IMPORTANT: Since the scope is set at render, you may want to customize it so that current_user isn't called on every request. This was also a problem in 0.9.
This causes that the current_user method is always invoked. If you want to avoid this behaviour, you can set the serialization_scope in the controller for example:
class UsersController < ApplicationController
serialization_scope nil # also you can pass a custom method here
def current
render json: { :errors => "Incorrect credentials" }, :status => :bad_request
end
end
or in some cases only by calling self.class.serialization_scope nil.