FIX: `Topic#fancy_title` should not write in readonly mode.

app/models/topic.rb

@@ -311,11 +311,10 @@ class Topic < ActiveRecord::Base
     return ERB::Util.html_escape(title) unless SiteSetting.title_fancy_entities?
 
     unless fancy_title = read_attribute(:fancy_title)
-
       fancy_title = Topic.fancy_title(title)
       write_attribute(:fancy_title, fancy_title)
 
-      unless new_record?
+      if !new_record? && !Discourse.readonly_mode?
         # make sure data is set in table, this also allows us to change algorithm
         # by simply nulling this column
         exec_sql("UPDATE topics SET fancy_title = :fancy_title where id = :id", id: self.id, fancy_title: fancy_title)

spec/models/topic_spec.rb

@@ -327,6 +327,27 @@ describe Topic do
         topic.title = "this is another edge case"
         expect(topic.fancy_title).to eq("this is another edge case")
       end
+
+      context 'readonly mode' do
+        before do
+          Discourse.enable_readonly_mode
+        end
+
+        after do
+          Discourse.disable_readonly_mode
+        end
+
+        it 'should not attempt to update `fancy_title`' do
+          topic.save!
+          expect(topic.fancy_title).to eq('“this topic” – has “fancy stuff”')
+
+          topic.title = "This is a test testing testing"
+          expect(topic.fancy_title).to eq("This is a test testing testing")
+
+          expect(topic.reload.read_attribute(:fancy_title))
+            .to eq('“this topic” – has “fancy stuff”')
+        end
+      end
     end
   end