Add migration that deletes additional application_settings rows

Add changelog entry

Add cr remarks

Add safeguard

Fix failing pipeline

Add cr remarks

changelogs/unreleased/214983-remove_additional_rows_in_application_settings.yml

+---
+title: Delete orphaned rows in application_settings table
+merge_request: 29981
+author:
+type: performance

db/post_migrate/20200420162730_remove_additional_application_settings_rows.rb

+# frozen_string_literal: true
+
+class RemoveAdditionalApplicationSettingsRows < ActiveRecord::Migration[6.0]
+  class ApplicationSetting < ActiveRecord::Base
+    self.table_name = 'application_settings'
+  end
+
+  def up
+    return if ApplicationSetting.count == 1
+
+    execute "DELETE from application_settings WHERE id NOT IN (SELECT MAX(id) FROM application_settings);"
+  end
+
+  def down
+    # no changes
+  end
+end

db/structure.sql

@@ -13421,6 +13421,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200416120128
 20200416120354
 20200417044453
+20200420162730
 20200420172113
 20200420172752
 20200420172927

spec/migrations/remove_additional_application_settings_rows_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require Rails.root.join('db', 'post_migrate', '20200420162730_remove_additional_application_settings_rows.rb')
+
+describe RemoveAdditionalApplicationSettingsRows do
+  let(:application_settings) { table(:application_settings) }
+
+  it 'removes additional rows from application settings' do
+    3.times { application_settings.create! }
+    latest_settings = application_settings.create!
+
+    disable_migrations_output { migrate! }
+
+    expect(application_settings.count).to eq(1)
+    expect(application_settings.first).to eq(latest_settings)
+  end
+
+  it 'leaves only row in application_settings' do
+    latest_settings = application_settings.create!
+
+    disable_migrations_output { migrate! }
+
+    expect(application_settings.first).to eq(latest_settings)
+  end
+end