fix: Use last_activity_at instead of updated_at for sorting (#1281)

Co-authored-by: Akash Srivastava <akash.srivastava.1911@gmail.com>

db/migrate/20200927135222_add_last_activity_at_to_conversation.rb

@@ -0,0 +1,35 @@
+class AddLastActivityAtToConversation < ActiveRecord::Migration[6.0]
+  def up
+    add_column :conversations,
+               :last_activity_at,
+               :datetime,
+               default: -> { 'CURRENT_TIMESTAMP' },
+               index: true
+
+    add_last_activity_at_to_conversations
+
+    change_column_null(:conversations, :last_activity_at, false)
+  end
+
+  def down
+    remove_column(:conversations, :last_activity_at)
+  end
+
+  private
+
+  def add_last_activity_at_to_conversations
+    ::Conversation.find_in_batches do |conversation_batch|
+      Rails.logger.info "Migrated till #{conversation_batch.first.id}\n"
+      conversation_batch.each do |conversation|
+        # rubocop:disable Rails/SkipsModelValidations
+        last_activity_at = if conversation.messages.last
+                             conversation.messages.last.created_at
+                           else
+                             conversation.created_at
+                           end
+        conversation.update_columns(last_activity_at: last_activity_at)
+        # rubocop:enable Rails/SkipsModelValidations
+      end
+    end
+  end
+end