Add epics.relative_position column if missing

If this column is missing for some reason, add it and
backfill it with epic.id (to assure it's not nil).

Changelog: added

db/migrate/20220407135820_add_epics_relative_position.rb

+# frozen_string_literal: true
+
+class AddEpicsRelativePosition < Gitlab::Database::Migration[1.0]
+  DOWNTIME = false
+
+  def up
+    return unless table_exists?(:epics)
+    return if column_exists?(:epics, :relative_position)
+
+    add_column :epics, :relative_position, :integer
+
+    execute('UPDATE epics SET relative_position=id*500')
+  end
+
+  def down
+    # no-op - this column should normally exist if epics table exists too
+  end
+end

db/schema_migrations/20220407135820

+ab7bb319a7099714d9863ec16b7dcf8c1aeab495b8635a01dff4a51fab876b6b
\ No newline at end of file

spec/migrations/add_epics_relative_position_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require_migration!
+
+RSpec.describe AddEpicsRelativePosition, :migration do
+  let(:groups) { table(:namespaces) }
+  let(:epics) { table(:epics) }
+  let(:users) { table(:users) }
+  let(:user) { users.create!(name: 'user', email: 'email@example.org', projects_limit: 100) }
+  let(:group) { groups.create!(name: 'gitlab', path: 'gitlab-org', type: 'Group') }
+
+  let!(:epic1) { epics.create!(title: 'epic 1', title_html: 'epic 1', author_id: user.id, group_id: group.id, iid: 1) }
+  let!(:epic2) { epics.create!(title: 'epic 2', title_html: 'epic 2', author_id: user.id, group_id: group.id, iid: 2) }
+  let!(:epic3) { epics.create!(title: 'epic 3', title_html: 'epic 3', author_id: user.id, group_id: group.id, iid: 3) }
+
+  it 'does nothing if epics table contains relative_position' do
+    expect { migrate! }.not_to change { epics.pluck(:relative_position) }
+  end
+
+  it 'adds relative_position if missing and backfills it with ID value', :aggregate_failures do
+    ActiveRecord::Base.connection.execute('ALTER TABLE epics DROP relative_position')
+
+    migrate!
+
+    expect(epics.pluck(:relative_position)).to match_array([epic1.id * 500, epic2.id * 500, epic3.id * 500])
+  end
+end