Add heartbeat model, next_heartbeat_time and update_heartbeat_time on device registration

app/models/device.rb

@@ -2,8 +2,19 @@ class Device < ActiveRecord::Base
   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
 
   def generate_access_token
     self.access_token = SecureRandom.uuid
   end
+
+  def update_last_heartbeats_time
+    self.last_heartbeat_recd_time = self.updated_at
+    self.save
+  end
+
+  def next_heartbeat_time
+    (self.last_heartbeat_recd_time + HEARTBEAT_INTERVAL).to_i
+  end
 end