Migration: add wiki_page_meta and wiki_page_slugs

This adds two new tables, `wiki_page_slugs` and `wiki_page_meta` to hold
metadata about wiki pages.

The string limit for titles is set at our application limit, and the
slug limit is set at the maximum valid URL length.

The rule that one slug at a time may be canonical for a given page is
enforced using a unique index.

Adds changelog describing this change.

changelogs/unreleased/30526-a-be-wiki-activity-Models.yml

+---
+title: Adds wiki metadata models
+merge_request: 26529
+author:
+type: added

db/migrate/20200302152516_add_wiki_slug.rb

+# frozen_string_literal: true
+
+class AddWikiSlug < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    create_table :wiki_page_meta, id: :serial do |t|
+      t.references :project, index: true, foreign_key: { on_delete: :cascade }, null: false
+      t.string :title, null: false, limit: 255
+    end
+
+    create_table :wiki_page_slugs, id: :serial do |t|
+      t.boolean :canonical, default: false, null: false
+      t.references :wiki_page_meta, index: true, foreign_key: { on_delete: :cascade }, null: false
+      t.string :slug, null: false, limit: 2048
+      t.index [:slug, :wiki_page_meta_id], unique: true
+      t.index [:wiki_page_meta_id], name: 'one_canonical_wiki_page_slug_per_metadata', unique: true, where: "(canonical = true)"
+    end
+  end
+end

db/schema.rb

@@ -4667,6 +4667,21 @@ ActiveRecord::Schema.define(version: 2020_03_13_123934) do
     t.index ["type"], name: "index_web_hooks_on_type"
   end
 
+  create_table "wiki_page_meta", id: :serial, force: :cascade do |t|
+    t.bigint "project_id", null: false
+    t.string "title", limit: 255, null: false
+    t.index ["project_id"], name: "index_wiki_page_meta_on_project_id"
+  end
+
+  create_table "wiki_page_slugs", id: :serial, force: :cascade do |t|
+    t.boolean "canonical", default: false, null: false
+    t.bigint "wiki_page_meta_id", null: false
+    t.string "slug", limit: 2048, null: false
+    t.index ["slug", "wiki_page_meta_id"], name: "index_wiki_page_slugs_on_slug_and_wiki_page_meta_id", unique: true
+    t.index ["wiki_page_meta_id"], name: "index_wiki_page_slugs_on_wiki_page_meta_id"
+    t.index ["wiki_page_meta_id"], name: "one_canonical_wiki_page_slug_per_metadata", unique: true, where: "(canonical = true)"
+  end
+
   create_table "x509_certificates", force: :cascade do |t|
     t.datetime_with_timezone "created_at", null: false
     t.datetime_with_timezone "updated_at", null: false
@@ -5211,6 +5226,8 @@ ActiveRecord::Schema.define(version: 2020_03_13_123934) do
   add_foreign_key "vulnerability_scanners", "projects", on_delete: :cascade
   add_foreign_key "web_hook_logs", "web_hooks", on_delete: :cascade
   add_foreign_key "web_hooks", "projects", name: "fk_0c8ca6d9d1", on_delete: :cascade
+  add_foreign_key "wiki_page_meta", "projects", on_delete: :cascade
+  add_foreign_key "wiki_page_slugs", "wiki_page_meta", column: "wiki_page_meta_id", on_delete: :cascade
   add_foreign_key "x509_certificates", "x509_issuers", on_delete: :cascade
   add_foreign_key "x509_commit_signatures", "projects", on_delete: :cascade
   add_foreign_key "x509_commit_signatures", "x509_certificates", on_delete: :cascade