Speed up checking whether fields are markdown fields or not

app/models/concerns/cache_markdown_field.rb

@@ -27,7 +27,7 @@ module CacheMarkdownField
   # Returns the default Banzai render context for the cached markdown field.
   def banzai_render_context(field)
     raise ArgumentError, "Unknown field: #{field.inspect}" unless
-      cached_markdown_fields.markdown_fields.include?(field)
+      cached_markdown_fields.key?(field)
 
     # Always include a project key, or Banzai complains
     project = self.project if self.respond_to?(:project)
@@ -100,7 +100,7 @@ module CacheMarkdownField
 
   def cached_html_for(markdown_field)
     raise ArgumentError, "Unknown field: #{markdown_field}" unless
-      cached_markdown_fields.markdown_fields.include?(markdown_field)
+      cached_markdown_fields.key?(markdown_field)
 
     __send__(cached_markdown_fields.html_field(markdown_field)) # rubocop:disable GitlabSecurity/PublicSend
   end
@@ -108,7 +108,7 @@ module CacheMarkdownField
   # Updates the markdown cache if necessary, then returns the field
   # Unlike `cached_html_for` it returns `nil` if the field does not exist
   def updated_cached_html_for(markdown_field)
-    return unless cached_markdown_fields.markdown_fields.include?(markdown_field)
+    return unless cached_markdown_fields.key?(markdown_field)
 
     if attribute_invalidated?(cached_markdown_fields.html_field(markdown_field))
       # Invalidated due to Markdown content change

lib/gitlab/markdown_cache/field_data.rb

@@ -9,7 +9,7 @@ module Gitlab
         @data = {}
       end
 
-      delegate :[], :[]=, to: :@data
+      delegate :[], :[]=, :key?, to: :@data
 
       def markdown_fields
         @data.keys

spec/lib/gitlab/markdown_cache/field_data_spec.rb

@@ -12,4 +12,11 @@ RSpec.describe Gitlab::MarkdownCache::FieldData do
   it 'translates a markdown field name into a html field name' do
     expect(field_data.html_field(:description)).to eq("description_html")
   end
+
+  describe '#key?' do
+    specify do
+      expect(field_data.key?(:description)).to be_truthy
+      expect(field_data.key?(:something_else)).to be_falsy
+    end
+  end
 end