BUGFIX: admins were unable to change the GL.com plan for a namespace

The GL.com plan is no longer stored with Namespace, now this attribute
is part of the new GitlabSubscription model.

ee/app/controllers/ee/admin/groups_controller.rb

@@ -21,7 +21,7 @@ module EE
         super + [
           :repository_size_limit,
           :shared_runners_minutes_limit,
-          :plan_id
+          gitlab_subscription_attributes: [:hosted_plan_id]
         ].tap do |params_ee|
           params_ee << :project_creation_level if @group&.feature_available?(:project_creation_level)
         end

ee/app/controllers/ee/admin/users_controller.rb

@@ -20,7 +20,11 @@ module EE
       def allowed_user_params
         super + [
           :note,
-          namespace_attributes: [:id, :shared_runners_minutes_limit, :plan_id]
+          namespace_attributes: [
+            :id,
+            :shared_runners_minutes_limit,
+            gitlab_subscription_attributes: [:hosted_plan_id]
+          ]
         ]
       end
     end

ee/app/models/ee/namespace.rb

@@ -33,6 +33,8 @@ module EE
       has_one :namespace_statistics
       has_one :gitlab_subscription, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 
+      accepts_nested_attributes_for :gitlab_subscription
+
       scope :with_plan, -> { where.not(plan_id: nil) }
 
       delegate :shared_runners_minutes, :shared_runners_seconds, :shared_runners_seconds_last_reset,

ee/app/models/gitlab_subscription.rb

 # frozen_string_literal: true
 
 class GitlabSubscription < ActiveRecord::Base
+  default_value_for :start_date, Date.today
+
   belongs_to :namespace
   belongs_to :hosted_plan, class_name: 'Plan'
 

ee/app/views/admin/_namespace_plan.html.haml

@@ -4,6 +4,7 @@
 .form-group.row
   = f.label :plan, class: 'col-form-label col-sm-2'
   .col-sm-10
-    = f.select :plan_id, Plan.pluck(:title, :id),
-      { include_blank: 'No plan' },
-      class: 'form-control'
+    = f.fields_for :gitlab_subscription do |f|
+      = f.select :hosted_plan_id, Plan.pluck(:title, :id),
+        { include_blank: 'No plan' },
+        class: 'form-control'

ee/spec/features/admin/groups/admin_changes_plan_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Changes GL.com plan for group' do
+  include WaitForRequests
+
+  let!(:silver_plan) { create(:silver_plan) }
+  let(:admin) { create(:admin) }
+
+  before do
+    allow(Gitlab::CurrentSettings).to receive(:should_check_namespace_plan?) { true }
+
+    sign_in(admin)
+  end
+
+  describe 'for group namespace' do
+    let(:group) { create(:group) }
+
+    before do
+      visit admin_group_path(group)
+      click_link 'Edit'
+    end
+
+    it 'changes the plan' do
+      find('#group_gitlab_subscription_attributes_hosted_plan_id').find(:xpath, 'option[2]').select_option
+
+      click_button('Save changes')
+
+      expect(page).to have_content('Plan: Silver')
+    end
+  end
+
+  describe 'for user namespace' do
+    let(:user) { create(:user) }
+
+    before do
+      visit admin_user_path(user)
+      click_link 'Edit'
+    end
+
+    it 'changes the plan' do
+      find('#user_namespace_attributes_gitlab_subscription_attributes_hosted_plan_id').find(:xpath, 'option[2]').select_option
+
+      click_button('Save changes')
+
+      expect(page).to have_content('Plan: Silver')
+    end
+  end
+end

ee/spec/models/gitlab_subscription_spec.rb

 require 'spec_helper'
 
 describe GitlabSubscription do
+  describe 'default values' do
+    it do
+      expect(subject.start_date).to eq(Date.today)
+    end
+  end
+
   describe 'validations' do
     it { is_expected.to validate_presence_of(:seats) }
     it { is_expected.to validate_presence_of(:start_date) }