Add migration and spec

Add migration to save Instance Administrators group ID in
application_settings table.

changelogs/unreleased/migration-save-instance-administrators-id.yml

+---
+title: Add migration to save Instance Administrators group ID in application_settings table
+merge_request: 24796
+author:
+type: changed

db/post_migrate/20200210092405_save_instance_administrators_group_id.rb

+# frozen_string_literal: true
+
+class SaveInstanceAdministratorsGroupId < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def up
+    execute(
+      <<-SQL
+      UPDATE
+        application_settings
+      SET
+        instance_administrators_group_id = (
+          SELECT
+            namespace_id
+          FROM
+            projects
+          WHERE
+            id = application_settings.instance_administration_project_id
+        )
+      WHERE
+        instance_administrators_group_id IS NULL
+        AND
+        instance_administration_project_id IS NOT NULL
+        AND
+        ID in (
+          SELECT
+            max(id)
+          FROM
+            application_settings
+        )
+      SQL
+    )
+  end
+
+  def down
+    # no-op
+
+    # The change performed by `up` cannot be reversed because once the migration runs,
+    # we do not know what value application_settings.instance_administrators_group_id
+    # had before the migration was run.
+  end
+end

spec/migrations/save_instance_administrators_group_id_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20200210092405_save_instance_administrators_group_id')
+
+describe SaveInstanceAdministratorsGroupId, :migration do
+  let(:application_settings_table) { table(:application_settings) }
+
+  let(:instance_administrators_group) do
+    table(:namespaces).create!(
+      id: 1,
+      name: 'GitLab Instance Administrators',
+      path: 'gitlab-instance-administrators-random',
+      type: 'Group'
+    )
+  end
+
+  let(:self_monitoring_project) do
+    table(:projects).create!(
+      id: 2,
+      name: 'Self Monitoring',
+      path: 'self_monitoring',
+      namespace_id: instance_administrators_group.id
+    )
+  end
+
+  context 'when project ID is saved but group ID is not' do
+    let(:application_settings) do
+      application_settings_table.create!(instance_administration_project_id: self_monitoring_project.id)
+    end
+
+    it 'saves instance administrators group ID' do
+      expect(application_settings.instance_administration_project_id).to eq(self_monitoring_project.id)
+      expect(application_settings.instance_administrators_group_id).to be_nil
+
+      migrate!
+
+      expect(application_settings.reload.instance_administrators_group_id).to eq(instance_administrators_group.id)
+      expect(application_settings.instance_administration_project_id).to eq(self_monitoring_project.id)
+    end
+  end
+
+  context 'when group ID is saved but project ID is not' do
+    let(:application_settings) do
+      application_settings_table.create!(instance_administrators_group_id: instance_administrators_group.id)
+    end
+
+    it 'does not make changes' do
+      expect(application_settings.instance_administrators_group_id).to eq(instance_administrators_group.id)
+      expect(application_settings.instance_administration_project_id).to be_nil
+
+      migrate!
+
+      expect(application_settings.reload.instance_administrators_group_id).to eq(instance_administrators_group.id)
+      expect(application_settings.instance_administration_project_id).to be_nil
+    end
+  end
+
+  context 'when group ID and project ID are both saved' do
+    let(:application_settings) do
+      application_settings_table.create!(
+        instance_administrators_group_id: instance_administrators_group.id,
+        instance_administration_project_id: self_monitoring_project.id
+      )
+    end
+
+    it 'does not make changes' do
+      expect(application_settings.instance_administrators_group_id).to eq(instance_administrators_group.id)
+      expect(application_settings.instance_administration_project_id).to eq(self_monitoring_project.id)
+
+      migrate!
+
+      expect(application_settings.reload.instance_administrators_group_id).to eq(instance_administrators_group.id)
+      expect(application_settings.instance_administration_project_id).to eq(self_monitoring_project.id)
+    end
+  end
+
+  context 'when neither group ID nor project ID is saved' do
+    let(:application_settings) do
+      application_settings_table.create!
+    end
+
+    it 'does not make changes' do
+      expect(application_settings.instance_administrators_group_id).to be_nil
+      expect(application_settings.instance_administration_project_id).to be_nil
+
+      migrate!
+
+      expect(application_settings.reload.instance_administrators_group_id).to be_nil
+      expect(application_settings.instance_administration_project_id).to be_nil
+    end
+  end
+
+  context 'when application_settings table has no rows' do
+    it 'does not fail' do
+      migrate!
+    end
+  end
+end