Adds event enum column to DesignsVersions join table

Also adds index to support scopes and queries

This supports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14656

changelogs/unreleased/delete-designs-v2.yml

+---
+title: Adds event enum column to DesignsVersions join table
+merge_request: 30745
+type: added

db/migrate/20190715140740_add_event_type_to_design_management_designs_versions.rb

+# frozen_string_literal: true
+
+# This migration sets up a event enum on the DesignsVersions join table
+class AddEventTypeToDesignManagementDesignsVersions < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # We disable these cops here because adding this column is safe. The table does not
+  # have any data in it.
+  # rubocop: disable Migration/AddIndex
+  # rubocop: disable Migration/AddColumn
+  def up
+    add_column(:design_management_designs_versions, :event, :integer,
+               limit: 2,
+               null: false,
+               default: 0)
+    add_index(:design_management_designs_versions, :event)
+  end
+
+  # rubocop: disable Migration/RemoveIndex
+  def down
+    remove_index(:design_management_designs_versions, :event)
+    remove_column(:design_management_designs_versions, :event)
+  end
+end

db/schema.rb

@@ -1097,8 +1097,10 @@ ActiveRecord::Schema.define(version: 2019_07_15_142138) do
   create_table "design_management_designs_versions", id: false, force: :cascade do |t|
     t.bigint "design_id", null: false
     t.bigint "version_id", null: false
+    t.integer "event", limit: 2, default: 0, null: false
     t.index ["design_id", "version_id"], name: "design_management_designs_versions_uniqueness", unique: true
     t.index ["design_id"], name: "index_design_management_designs_versions_on_design_id"
+    t.index ["event"], name: "index_design_management_designs_versions_on_event"
     t.index ["version_id"], name: "index_design_management_designs_versions_on_version_id"
   end