Fix encoding issue with enums

Using the result from `build_from_database` and passing it to
the `attributes` from init_with` was not sufficent to decode enums
correctly. Since `init_with` is guaranteed to work with `encode_with`,
this MR uses now `encode_with` instead to fix the encoding issue.

lib/gitlab/json_cache.rb

@@ -80,15 +80,10 @@ module Gitlab
       # when the new_record? method incorrectly returns false.
       #
       # See https://gitlab.com/gitlab-org/gitlab/issues/9903#note_145329964
-      klass
-        .allocate
-        .init_with(
-          "attributes" => attributes_for(klass, raw),
-          "new_record" => new_record?(raw, klass)
-        )
+      klass.allocate.init_with(encode_for(klass, raw))
     end
 
-    def attributes_for(klass, raw)
+    def encode_for(klass, raw)
       # We have models that leave out some fields from the JSON export for
       # security reasons, e.g. models that include the CacheMarkdownField.
       # The ActiveRecord::AttributeSet we build from raw does know about
@@ -96,7 +91,10 @@ module Gitlab
       missing_attributes = (klass.columns.map(&:name) - raw.keys)
       missing_attributes.each { |column| raw[column] = nil }
 
-      klass.attributes_builder.build_from_database(raw, {})
+      coder = {}
+      klass.new(raw).encode_with(coder)
+      coder["new_record"] = new_record?(raw, klass)
+      coder
     end
 
     def new_record?(raw, klass)

spec/lib/gitlab/json_cache_spec.rb

@@ -378,6 +378,12 @@ describe Gitlab::JsonCache do
           expect(result).to eq(broadcast_message)
         end
 
+        it 'decodes enums correctly' do
+          result = cache.fetch(key, as: BroadcastMessage) { 'block result' }
+
+          expect(result.broadcast_type).to eq(broadcast_message.broadcast_type)
+        end
+
         context 'when the cached value is an instance of ActiveRecord::Base' do
           it 'returns a persisted record when id is set' do
             backend.write(expanded_key, broadcast_message.to_json)