Minor improvements

core/src/main/scala/lightdb/model/RecordDocumentCollection.scala

@@ -0,0 +1,5 @@
+package lightdb.model
+
+import lightdb.{LightDB, RecordDocument}
+
+abstract class RecordDocumentCollection[D <: RecordDocument[D]](collectionName: String, db: LightDB) extends Collection[D](collectionName, db) with RecordDocumentModel[D]

core/src/main/scala/lightdb/model/RecordDocumentModel.scala

@@ -0,0 +1,13 @@
+package lightdb.model
+
+import cats.effect.IO
+import fabric.Json
+import lightdb.RecordDocument
+
+trait RecordDocumentModel[D <: RecordDocument[D]] extends DocumentModel[D] {
+  override def preSetJson(json: Json, collection: AbstractCollection[D]): IO[Json] = IO {
+    json.modify("modified") { _ =>
+      System.currentTimeMillis()
+    }
+  }
+}