Disable creating user mentions during import

Creating a note causes a subtransaction every time `CacheMarkdownField`
attempts to refresh the issuable's user mentions. We disable it for now
to avoid large number of SQL queries. We are not using this table at the
moment for reads. https://gitlab.com/gitlab-org/gitlab/-/issues/21801
tracks the work necessary to make this work.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/336788

Changelog: performance

lib/gitlab/markdown_cache/active_record/extension.rb

@@ -10,7 +10,9 @@ module Gitlab
           # Using before_update here conflicts with elasticsearch-model somehow
           before_create :refresh_markdown_cache, if: :invalidated_markdown_cache?
           before_update :refresh_markdown_cache, if: :invalidated_markdown_cache?
-          after_save :store_mentions!, if: :mentionable_attributes_changed?
+          # The import case needs to be fixed to avoid large number of
+          # SQL queries: https://gitlab.com/gitlab-org/gitlab/-/issues/21801
+          after_save :store_mentions!, if: :mentionable_attributes_changed?, unless: ->(obj) { obj.is_a?(Importable) && obj.importing? }
         end
 
         # Always exclude _html fields from attributes (including serialization).

spec/lib/gitlab/markdown_cache/active_record/extension_spec.rb

@@ -178,4 +178,42 @@ RSpec.describe Gitlab::MarkdownCache::ActiveRecord::Extension do
       thing.refresh_markdown_cache!
     end
   end
+
+  context 'with note' do
+    let(:klass) do
+      Class.new(ActiveRecord::Base) do
+        self.table_name = 'notes'
+        include CacheMarkdownField
+        include Importable
+        include Mentionable
+
+        attr_mentionable :note, pipeline: :note
+        cache_markdown_field :note, pipeline: :note
+      end
+    end
+
+    let(:thing) { klass.new(note: markdown) }
+
+    before do
+      thing.note = "hello world"
+    end
+
+    it 'calls store_mentions!' do
+      expect(thing).to receive(:store_mentions!).and_call_original
+
+      thing.save!
+    end
+
+    context 'during import' do
+      before do
+        thing.importing = true
+      end
+
+      it 'does not call store_mentions!' do
+        expect(thing).not_to receive(:store_mentions!)
+
+        thing.save!
+      end
+    end
+  end
 end