Add wiki page metadata models

Adds wiki page slug model:

A slug represents a historical set of slugs that identify wiki pages. At
most one can be canonical at any one time, per page.

Adds wiki page meta model:

The wiki page metadata model contains the current title, and has access
to a collection of slugs, at most one of which may be canonical at any
one point (all others are historic).

Rather than polluting the model namespace, and to clarify the meaning of
the class names, these classes are placed under the `WikiPage`
namespace.

Adds adds wiki page meta/spec factories with sequences for use in the
specs, guaranteeing unique titles and slugs

app/models/wiki_page.rb

@@ -29,6 +29,12 @@ class WikiPage
 
   alias_method :==, :eql?
 
+  def meta
+    raise 'Metadata only available for valid pages' unless valid?
+
+    @meta ||= WikiPage::Meta.find_or_create(slug, self)
+  end
+
   # Sorts and groups pages by directory.
   #
   # pages - an array of WikiPage objects.
@@ -236,6 +242,7 @@ class WikiPage
     end
 
     save do
+      @meta = nil if title_changed?
       wiki.update_page(
         @page,
         content: content,

app/models/wiki_page/meta.rb

+# frozen_string_literal: true
+
+class WikiPage
+  class Meta < ApplicationRecord
+    include Gitlab::Utils::StrongMemoize
+
+    self.table_name = 'wiki_page_meta'
+
+    belongs_to :project
+
+    has_many :slugs, class_name: 'WikiPage::Slug', foreign_key: 'wiki_page_meta_id', inverse_of: :wiki_page_meta
+    has_many :events, as: :target, dependent: :delete_all # rubocop:disable Cop/ActiveRecordDependent
+
+    validates :title, presence: true
+    validates :project_id, presence: true
+
+    alias_method :resource_parent, :project
+
+    # Return the (updated) WikiPage::Meta record for a given wiki page
+    #
+    # If none is found, then a new record is created, and its fields are set
+    # to reflect the wiki_page passed.
+    #
+    # @param [String] last_known_slug
+    # @param [WikiPage] wiki_page
+    #
+    # As with all `find_or_create` methods, this one raises errors on
+    # validation issues.
+    def self.find_or_create(last_known_slug, wiki_page)
+      project = wiki_page.wiki.project
+
+      meta = find_by_canonical_slug(last_known_slug, project) || create(title: wiki_page.title, project_id: project.id)
+
+      meta.update_wiki_page_attributes(wiki_page)
+      meta.insert_slugs([last_known_slug, wiki_page.slug], wiki_page.slug)
+      meta.canonical_slug = wiki_page.slug
+
+      meta
+    end
+
+    def update_wiki_page_attributes(page)
+      update_column(:title, page.title) unless page.title == title
+    end
+
+    def insert_slugs(strings, canonical)
+      slug_attrs = strings.uniq.map do |slug|
+        { wiki_page_meta_id: id, slug: slug }
+      end
+      slugs.insert_all(slug_attrs)
+    end
+
+    def self.find_by_canonical_slug(canonical_slug, project)
+      meta = joins(:slugs).find_by(project_id: project.id,
+                                   wiki_page_slugs: { canonical: true, slug: canonical_slug })
+
+      # Prevent queries for canonical_slug
+      meta.instance_variable_set(:@canonical_slug, canonical_slug) if meta
+
+      meta
+    end
+
+    def canonical_slug
+      strong_memoize(:canonical_slug) { slugs.canonical.first&.slug }
+    end
+
+    def canonical_slug=(slug)
+      return if @canonical_slug == slug
+
+      if persisted?
+        slugs.update_all(canonical: false)
+        page_slug = slugs.create_with(canonical: true).find_or_create_by(slug: slug)
+        page_slug.update_column(:canonical, true) unless page_slug.canonical?
+      else
+        slugs.new(slug: slug, canonical: true)
+      end
+
+      @canonical_slug = slug
+    end
+  end
+end

app/models/wiki_page/slug.rb

+# frozen_string_literal: true
+
+class WikiPage
+  class Slug < ApplicationRecord
+    self.table_name = 'wiki_page_slugs'
+
+    belongs_to :wiki_page_meta, class_name: 'WikiPage::Meta', inverse_of: :slugs
+
+    validates :slug, presence: true, uniqueness: { scope: :wiki_page_meta_id }
+    validate :only_one_slug_can_be_canonical_per_meta_record
+
+    scope :canonical, -> { where(canonical: true) }
+
+    private
+
+    def only_one_slug_can_be_canonical_per_meta_record
+      return unless canonical?
+
+      if other_slugs.canonical.exists?
+        errors.add(:canonical, 'Only one slug can be canonical per wiki metadata record')
+      end
+    end
+
+    def other_slugs
+      self.class.unscoped.where(wiki_page_meta_id: wiki_page_meta_id)
+    end
+  end
+end

spec/models/wiki_page/slug_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe WikiPage::Slug do
+  let_it_be(:meta) { create(:wiki_page_meta) }
+
+  describe 'Associations' do
+    it { is_expected.to belong_to(:wiki_page_meta) }
+
+    it 'refers correctly to the wiki_page_meta' do
+      created = create(:wiki_page_slug, wiki_page_meta: meta)
+
+      expect(created.reload.wiki_page_meta).to eq(meta)
+    end
+  end
+
+  describe 'scopes' do
+    describe 'canonical' do
+      subject { described_class.canonical }
+
+      context 'there are no slugs' do
+        it { is_expected.to be_empty }
+      end
+
+      context 'there are some non-canonical slugs' do
+        before do
+          create_list(:wiki_page_slug, 3)
+        end
+
+        it { is_expected.to be_empty }
+      end
+
+      context 'there is at least one canonical slugs' do
+        before do
+          create(:wiki_page_slug, canonical: true)
+        end
+
+        it { is_expected.not_to be_empty }
+      end
+    end
+  end
+
+  describe 'Validations' do
+    let(:canonical) { false }
+
+    subject do
+      build(:wiki_page_slug, canonical: canonical, wiki_page_meta: meta)
+    end
+
+    it { is_expected.to validate_presence_of(:slug) }
+    it { is_expected.to validate_uniqueness_of(:slug).scoped_to(:wiki_page_meta_id) }
+
+    describe 'only_one_slug_can_be_canonical_per_meta_record' do
+      context 'there are no other slugs' do
+        it { is_expected.to be_valid }
+
+        context 'the current slug is canonical' do
+          let(:canonical) { true }
+
+          it { is_expected.to be_valid }
+        end
+      end
+
+      context 'there are other slugs, but they are not canonical' do
+        before do
+          create_list(:wiki_page_slug, 3, wiki_page_meta: meta)
+        end
+
+        it { is_expected.to be_valid }
+
+        context 'the current slug is canonical' do
+          let(:canonical) { true }
+
+          it { is_expected.to be_valid }
+        end
+      end
+
+      context 'there is already a canonical slug' do
+        before do
+          create(:wiki_page_slug, canonical: true, wiki_page_meta: meta)
+        end
+
+        it { is_expected.to be_valid }
+
+        context 'the current slug is canonical' do
+          let(:canonical) { true }
+
+          it { is_expected.not_to be_valid }
+        end
+      end
+    end
+  end
+end