better migrations for unread count field on group accounts table

db/migrate/20190721214831_add_unread_count_to_group_accounts.rb

@@ -1,5 +1,10 @@
 class AddUnreadCountToGroupAccounts < ActiveRecord::Migration[5.2]
-  def change
-    add_column :group_accounts, :unread_count, :integer, default: 0
+  def up
+    add_column :group_accounts, :unread_count, :integer
+    change_column_default :group_accounts, :unread_count, 0
+  end
+
+  def down
+    remove_column :group_accounts, :unread_count
   end
 end

db/migrate/20190721234917_backfill_add_unread_count_to_group_accounts.rb

@@ -0,0 +1,10 @@
+class BackfillAddUnreadCountToGroupAccounts < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    GroupAccount.in_batches do |relation|
+      relation.update_all unread_count: 0
+      sleep(0.1)
+    end
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_07_21_214831) do
+ActiveRecord::Schema.define(version: 2019_07_21_234917) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"