Enough already. I'm obviously not incentivized/shamed into writing unit tests for things in UNTESTED dir.s. It's just making a mess of the file tree

DataLayer/TagPersistenceInterceptor.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Dinah.Core.Collections.Generic;
+using Dinah.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore;
+
+namespace DataLayer
+{
+    internal class TagPersistenceInterceptor : IDbInterceptor
+    {
+        public void Executed(DbContext context) { }
+
+        public void Executing(DbContext context)
+		{
+			var tagsCollection
+				= context
+				.ChangeTracker
+				.Entries()
+				.Where(e => e.State.In(EntityState.Modified, EntityState.Added))
+				.Select(e => e.Entity as UserDefinedItem)
+				.Where(udi => udi != null)
+				// do NOT filter out entires with blank tags. blank is the valid way to show the absence of tags
+				.Select(t => (t.Book.AudibleProductId, t.Tags))
+				.ToList();
+
+			FileManager.TagsPersistence.Save(tagsCollection);
+		}
+	}
+}