Change the registration API to return next heartbeat time

app/controllers/devices_controller.rb

@@ -6,7 +6,10 @@ class DevicesController < ApplicationController
     device = Device.find_or_initialize_by(unique_id: device_params[:unique_id])
     device.assign_attributes(device_params)
     if device.save
-      render json: { access_token: device.access_token }, status: :created
+      render json: { access_token: device.access_token,
+                     next_heartbeat_time: device.next_heartbeat_time
+                   }, 
+            status: :created
     else
       render json: { error: device.errors.full_messages }, status: :unprocessable_entity
     end

spec/controllers/devices_controller_spec.rb

@@ -6,6 +6,8 @@ RSpec.describe DevicesController, type: :controller do
     it "creates a device" do
       expect do
         post :create, device: attributes_for(:device), format: :json
+        expect(JSON.parse(response.body)["access_token"]).not_to be_nil
+        expect(JSON.parse(response.body)["next_heartbeat_time"]).not_to be_nil
       end.to change{ Device.count }.by(1)
     end