Add next_id field to timestamp record

lib/glueby/contract/active_record/timestamp.rb

@@ -9,7 +9,12 @@ class Timestamp < ::ActiveRecord::Base
 
         attr_reader :tx
 
-        belongs_to :prev, class_name: 'Glueby::Contract::AR::Timestamp', optional: true
+        belongs_to :prev, class_name: 'Glueby::Contract::AR::Timestamp', inverse_of: :next, optional: true
+        has_one :next, class_name: 'Glueby::Contract::AR::Timestamp', foreign_key: 'prev_id'
+
+        def next_id
+          self.next&.id
+        end
 
         validate :validate_prev
 
@@ -114,6 +119,7 @@ def save_with_broadcast!(fee_estimator: Glueby::Contract::FeeEstimator::Fixed.ne
 
               if update_trackable?
                 prev.latest = false
+                prev.next = self
                 prev.save!
               end
             end

spec/glueby/contract/timestamp/active_record/timestamp_spec.rb

@@ -519,11 +519,14 @@
             expect(timestamp.p2c_address).not_to be_nil
             expect(timestamp.payment_base).not_to be_nil
             expect(timestamp.latest).to be_truthy
+            expect(timestamp.next_id).to be_nil
           end
 
-          it 'update previous timestamp\'s latest falg to false' do
+          it 'update previous timestamp\'s latest falg and next_id' do
             subject
-            expect(prev.reload.latest).to be_falsey
+            prev.reload
+            expect(prev.latest).to be_falsey
+            expect(prev.next_id).to eq timestamp.id
           end
 
           context 'previous timestamp type is not trackable' do