Reuse epoch-verified record snapshots and avoid empty push payloads

app/models/record_cache.rb

@@ -0,0 +1,24 @@
+# Rebuild request-local models from bounded, immutable database snapshots.
+# The observer must still see the captured epoch after lookup and admission.
+class RecordCache
+  def self.fetch(key, version)
+    return yield unless version && ResponseCache.instance.budget.positive? && !ActiveRecord::Base.connection.transaction_open?
+
+    key = ActiveSupport::Cache.expand_cache_key([ "record-snapshot-v1", version, key ])
+    return yield if key.bytesize > ResponseCache::MAX_KEY_BYTES
+
+    if snapshot = FragmentCache.store.read(key)
+      if ResponseCache.instance.version == version
+        return ActiveSupport::JSON.decode(snapshot).map { |name, attributes| name.constantize.instantiate(attributes) }
+      end
+    end
+
+    records = yield
+    if records.all? { |record| record&.persisted? && !record.changed? } && ResponseCache.instance.version == version
+      # Raw database values retain timestamp precision and native enum binding.
+      snapshot = ActiveSupport::JSON.encode(records.map { |record| [ record.class.name, record.attributes_before_type_cast ] })
+      FragmentCache.store.write(key, snapshot) if snapshot.bytesize <= ResponseCache::MAX_ENTRY_BYTES
+    end
+    records
+  end
+end