Add seats related columns for easier data analysis

Don't keep history for these new columns since they can be calculated

changelogs/unreleased/store-seats_currently_in_use.yml

+---
+title: Add seats related columns for easier data analysis
+merge_request: 40470
+author:
+type: other

db/migrate/20200827085101_add_seats_currently_in_use_in_gitlab_subscriptions.rb

+# frozen_string_literal: true
+
+class AddSeatsCurrentlyInUseInGitlabSubscriptions < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      add_column :gitlab_subscriptions, :seats_in_use, :integer, default: 0, null: false
+      add_column :gitlab_subscriptions, :seats_owed, :integer, default: 0, null: false
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_column :gitlab_subscriptions, :seats_in_use
+      remove_column :gitlab_subscriptions, :seats_owed
+    end
+  end
+end

db/schema_migrations/20200827085101

+bdcfd7dbccc73d6e2754d67bf099cc2dd2b919e4125cec47e853ff91911cc82e
\ No newline at end of file

db/structure.sql

@@ -12133,7 +12133,9 @@ CREATE TABLE public.gitlab_subscriptions (
     seats integer DEFAULT 0,
     trial boolean DEFAULT false,
     trial_starts_on date,
-    auto_renew boolean
+    auto_renew boolean,
+    seats_in_use integer DEFAULT 0 NOT NULL,
+    seats_owed integer DEFAULT 0 NOT NULL
 );
 
 CREATE SEQUENCE public.gitlab_subscriptions_id_seq

ee/app/models/gitlab_subscription.rb

@@ -99,7 +99,7 @@ class GitlabSubscription < ApplicationRecord
     attrs['gitlab_subscription_id'] = self.id
     attrs['change_type'] = change_type
 
-    omitted_attrs = %w(id created_at updated_at)
+    omitted_attrs = %w(id created_at updated_at seats_in_use seats_owed)
 
     GitlabSubscriptionHistory.create(attrs.except(*omitted_attrs))
   end

ee/spec/models/gitlab_subscription_spec.rb

@@ -324,7 +324,7 @@ RSpec.describe GitlabSubscription do
     end
 
     it 'gitlab_subscription columns are contained in gitlab_subscription_history columns' do
-      diff_attrs = %w(updated_at)
+      diff_attrs = %w(updated_at seats_in_use seats_owed)
       expect(described_class.attribute_names - GitlabSubscriptionHistory.attribute_names).to eq(diff_attrs)
     end