Rspec for testing the rendered template outside of controller/view - ruby-on-rails

Question

I have a situation to render a HTML template outside of controller(A class under service/lib directory) and I am rendering the template using the below code.
class SomeClass
def some_method
#template = ApplicationController.render(
template: 'template',
layout: mailer_template,
)
end
end
Is there any ways to test if the rendered template is the expected one and whether render happened during that method call?
EDIT
class BatchSendingService < AbstractController::Base
require 'abstract_controller'
include AbstractController::Rendering
include AbstractController::AssetPaths
include AbstractController::Helpers
include Rails.application.routes.url_helpers
include ActionView::Rendering
include ActionView::ViewPaths
include ActionView::Layouts
self.view_paths = "app/views"
def send_batch_email(mail, domain)
#project = mail.project
#client = Mailgun::Client.new ENV['MAILGUN_API_KEY']
batch_message = Mailgun::BatchMessage.new(#client, domain)
batch_message.from(from_data)
mailer_layout = get_mailer_layout(mail.layout)
mail_html = render(
template: 'send_batch_email',
layout: mailer_layout
)
batch_message.body_html(mail_html.to_s)
batch_message.add_recipient(:to, recipient_email, {})
response = batch_message.finalize
end
EDIT
obj= BatchSendingService.new
allow(obj).to receive(:render)
BatchSendingService.send_batch_email(mail, domain)
expect(obj) .to have_received(:render)
.with({ template: "template", layout: "layout" })
By using the class where the instance method is called, the error is gone.

Answer 1

ActionController.render is a well tested method. The Rails Core Team saw to that. There's no need to test that it does what it says it does.
Rather, what you want to do is to make sure you called ActionController.render with the right parameters, using mock objects, like this:
describe SomeClass do
subject(:some_class) { described_class.new }
describe '#some_method' do
let(:template) { 'template' }
let(:layout) { 'mailer_template' }
before do
allow(ActionController).to receive(:render)
some_class.some_method
end
it 'renders the correct template' do
expect(ActionController)
.to have_received(:render)
.with({ template: template, layout: layout })
end
end
end
EDIT
Given the edited post, here's how I would approach the test. Note that not all of the code in your send_batch_email method is visible in your edit. So, YMMV:
describe BatchSendingService do
subject(:batch_sending_service) { described_class.new }
describe '#send_batch_email' do
subject(:send_batch_email) do
batch_sending_service.send_batch_email(email, domain)
end
let(:email) { 'email' }
let(:domain) { 'domain' }
let(:batch_message) do
instance_double(
Mailgun::BatchMessage,
from: true,
body_html: true,
add_recipient, true,
finalize: true
)
end
let(:template) { 'send_batch_template' }
let(:layout) { 'layout' }
before do
allow(Mailgun::Client).to receive(:new)
allow(Mailgun::BatchMessage)
.to receive(:new)
.and_return(batch_message)
allow(batch_sending_service)
.to receive(:render)
send_batch_email
end
it 'renders the correct template' do
expect(batch_sending_service)
.to have_received(:render)
.with(template, layout)
end
end
end