Clearer separation of concerns between page and meta

This makes the assumption that a page is valid an error condition, and
moves a long chain of accessors to the page model.

app/models/wiki_page.rb

@@ -295,6 +295,10 @@ class WikiPage
     'wiki_page'
   end
 
+  def version_commit_timestamp
+    version&.commit&.committed_date
+  end
+
   private
 
   def serialize_front_matter(hash)

app/models/wiki_page/meta.rb

@@ -5,6 +5,7 @@ class WikiPage
     include Gitlab::Utils::StrongMemoize
 
     CanonicalSlugConflictError = Class.new(ActiveRecord::RecordInvalid)
+    WikiPageInvalid = Class.new(ArgumentError)
 
     self.table_name = 'wiki_page_meta'
 
@@ -32,12 +33,13 @@ class WikiPage
       # @param [String] last_known_slug
       # @param [WikiPage] wiki_page
       #
-      # As with all `find_or_create` methods, this one raises errors on
-      # validation issues.
+      # This method raises errors on validation issues.
       def find_or_create(last_known_slug, wiki_page)
+        raise WikiPageInvalid unless wiki_page.valid?
+
         project = wiki_page.wiki.project
         known_slugs = [last_known_slug, wiki_page.slug].compact.uniq
-        raise 'no slugs!' if known_slugs.empty?
+        raise 'No slugs found! This should not be possible.' if known_slugs.empty?
 
         transaction do
           updates = wiki_page_updates(wiki_page)
@@ -70,7 +72,8 @@ class WikiPage
       private
 
       def wiki_page_updates(wiki_page)
-        last_commit_date = wiki_page.version.commit.committed_date
+        last_commit_date = wiki_page.version_commit_timestamp || Time.now.utc
+
         {
           title: wiki_page.title,
           created_at: last_commit_date,

spec/models/wiki_page/meta_spec.rb

@@ -233,6 +233,14 @@ describe WikiPage::Meta do
           expect { find_record }.to raise_error(ActiveRecord::RecordInvalid)
         end
       end
+
+      context 'the wiki page is not valid' do
+        let(:wiki_page) { build(:wiki_page, project: project, title: nil) }
+
+        it 'raises an error' do
+          expect { find_record }.to raise_error(described_class::WikiPageInvalid)
+        end
+      end
     end
 
     context 'no existing record exists' do

spec/models/wiki_page_spec.rb

@@ -844,6 +844,20 @@ describe WikiPage do
     end
   end
 
+  describe '#version_commit_timestamp' do
+    context 'for a new page' do
+      it 'returns nil' do
+        expect(new_page.version_commit_timestamp).to be_nil
+      end
+    end
+
+    context 'for page that exists' do
+      it 'returns the timestamp of the commit' do
+        expect(existing_page.version_commit_timestamp).to eq(existing_page.version.commit.committed_date)
+      end
+    end
+  end
+
   private
 
   def get_slugs(page_or_dir)