Fix last_value record in internal_ids for epics

The internal_ids last_value got out of sync with max(iid) for epics on
a couple groups, which prevents crearing new epics due to duplicate
iid that fails a unique index constrant, so we bring the last_value
up to date with max(iid) for epics on given groups

changelogs/unreleased/update-internal-ids-last-value-for-epics.yml

+---
+title: Fix last_value record in internal_ids for epics
+merge_request: 48988
+author:
+type: fixed

db/migrate/20201202081429_update_internal_ids_last_value_for_epics.rb

+# frozen_string_literal: true
+
+class UpdateInternalIdsLastValueForEpics < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def up
+    ApplicationRecord.connection.execute(<<-SQL.squish)
+      UPDATE internal_ids
+      SET last_value = epics_max_iids.maximum_iid
+      FROM
+        (
+          SELECT
+            MAX(epics.iid) AS maximum_iid,
+            epics.group_id AS epics_group_id
+          FROM epics
+          GROUP BY epics.group_id
+        ) epics_max_iids
+      WHERE internal_ids.last_value < epics_max_iids.maximum_iid
+        AND namespace_id = epics_max_iids.epics_group_id
+        AND internal_ids.usage = 4
+    SQL
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20201202081429

+cbc6bfa122167e9a46edaa14351a73eeb10586fa0eb82f231c792384c9d7986c
\ No newline at end of file

spec/migrations/update_internal_ids_last_value_for_epics_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20201202081429_update_internal_ids_last_value_for_epics.rb')
+
+RSpec.describe UpdateInternalIdsLastValueForEpics, :migration, schema: 20201124185639 do
+  let(:namespaces) { table(:namespaces) }
+  let(:users) { table(:users) }
+  let(:epics) { table(:epics) }
+  let(:internal_ids) { table(:internal_ids) }
+
+  let!(:author) { users.create!(name: 'test', email: 'test@example.com', projects_limit: 0) }
+  let!(:group1) { namespaces.create!(type: 'Group', name: 'group1', path: 'group1') }
+  let!(:group2) { namespaces.create!(type: 'Group', name: 'group2', path: 'group2') }
+  let!(:group3) { namespaces.create!(type: 'Group', name: 'group3', path: 'group3') }
+  let!(:epic_last_value1) { internal_ids.create!(usage: 4, last_value: 5, namespace_id: group1.id) }
+  let!(:epic_last_value2) { internal_ids.create!(usage: 4, last_value: 5, namespace_id: group2.id) }
+  let!(:epic_last_value3) { internal_ids.create!(usage: 4, last_value: 5, namespace_id: group3.id) }
+  let!(:epic_1) { epics.create!(iid: 110, title: 'from epic 1', group_id: group1.id, author_id: author.id, title_html: 'any') }
+  let!(:epic_2) { epics.create!(iid: 5, title: 'from epic 1', group_id: group2.id, author_id: author.id, title_html: 'any') }
+  let!(:epic_3) { epics.create!(iid: 3, title: 'from epic 1', group_id: group3.id, author_id: author.id, title_html: 'any') }
+
+  it 'updates out of sync internal_ids last_value' do
+    migrate!
+
+    expect(internal_ids.find_by(usage: 4, namespace_id: group1.id).last_value).to eq(110)
+    expect(internal_ids.find_by(usage: 4, namespace_id: group2.id).last_value).to eq(5)
+    expect(internal_ids.find_by(usage: 4, namespace_id: group3.id).last_value).to eq(5)
+  end
+end