Display status in admin console, Change the filters to include relevant fields, scopes for device status

app/admin/device.rb

@@ -17,10 +17,22 @@ ActiveAdmin.register Device do
   index do
     selectable_column
     id_column
+    column "Status" do | device |
+      status = device.status
+      status_tag status.titleize, STATUS_CLASSES[status.to_sym]
+    end    
     column "Model Name",:model
     column :created_at
     column :updated_at
+    column :last_heartbeat_recd_time
     actions
   end
+  filter :model
+  filter :created_at
+  filter :updated_at
+  filter :last_heartbeat_recd_time
 
+  scope :active
+  scope :missing
+  scope :dead
 end

app/models/device.rb

@@ -1,10 +1,20 @@
 class Device < ActiveRecord::Base
+  enum status: [:active,:missing,:dead]
+
+  attr_accessor :status
+  
   validates :unique_id, :model, presence: true
   validates :unique_id, uniqueness: true
+  
   before_create :generate_access_token
   after_create :update_last_heartbeats_time
+  
   has_many :heartbeats, dependent: :destroy
-
+  
+  scope :active, -> {where("last_heartbeat_recd_time > '#{Time.now.utc - ACTIVE_TIMEFRAME}'")}
+  scope :missing, -> {where("last_heartbeat_recd_time < '#{Time.now.utc - ACTIVE_TIMEFRAME}'AND last_heartbeat_recd_time > '#{Time.now.utc - MISSING_TIMEFRAME}'")}
+  scope :dead, -> {where("last_heartbeat_recd_time < '#{Time.now.utc - MISSING_TIMEFRAME}'")}
+  
   def generate_access_token
     self.access_token = SecureRandom.uuid
   end
@@ -17,4 +27,15 @@ class Device < ActiveRecord::Base
   def next_heartbeat_time
     (self.last_heartbeat_recd_time + HEARTBEAT_INTERVAL).to_i
   end
+
+  def status
+    time_elapsed = Time.now - self.last_heartbeat_recd_time
+    if time_elapsed < ACTIVE_TIMEFRAME
+      self.status = Device.statuses.keys[0]
+    elsif time_elapsed < MISSING_TIMEFRAME
+      self.status = Device.statuses.keys[1]
+    else
+      self.status = Device.statuses.keys[2]
+    end
+  end
 end