* reuse the first entity across attached entities that matching with …

…the same `UniqueLatestReplier` to re-achieve c8f4920

* fix entities that already exists are still attached in `DbContext.ChangeTracker`
+ local function `DetachAndReplace()`
@ `ThreadLatestReplierSaver.SaveFromThread()`

- field `_latestRepliersKeyByUnique` to let `FillFromRequestingWith602()` directly assign new instances of related entity `LatestReplier` @ ThreadCrawlFacade.cs
@ c#/crawler

c#/crawler/src/Tieba/Crawl/Facade/ThreadCrawlFacade.cs

@@ -14,8 +14,6 @@ public class ThreadCrawlFacade(
         postParser, postSaverFactory.Invoke,
         userParserFactory.Invoke, userSaverFactory.Invoke)
 {
-    private readonly Dictionary<ThreadLatestReplierSaver.UniqueLatestReplier, LatestReplier?> _latestRepliersKeyByUnique = [];
-
     public delegate ThreadCrawlFacade New(Fid fid, string forumName);
 
     protected override void OnPostParse(
@@ -44,21 +42,17 @@ join parsed in Posts.Values on (Tid)inResponse.Tid equals parsed.Tid
             { // replace with more detailed location.name in the 6.0.2 response
                 t.parsed.Geolocation = Helper.SerializedProtoBufOrNullIfEmpty(t.inResponse.Location);
             }
-            var name = t.inResponse.LastReplyer.Name.NullIfEmpty();
-            var nameShow = t.inResponse.LastReplyer.NameShow.NullIfEmpty();
+            var lastReplyer = t.inResponse.LastReplyer;
+            var name = lastReplyer?.Name.NullIfEmpty();
+            var nameShow = lastReplyer?.NameShow.NullIfEmpty();
 
             // LastReplyer will be null when LivePostType != "", but LastTimeInt will have expected timestamp value
-            var latestReplierEntity = t.inResponse.LastReplyer == null ? null : new LatestReplier
+            t.parsed.LatestReplier = lastReplyer == null ? null : new LatestReplier
             {
                 Name = name,
 #pragma warning disable S3358 // Ternary operators should not be nested
                 DisplayName = name == nameShow ? null : nameShow
 #pragma warning restore S3358 // Ternary operators should not be nested
             };
-            var uniqueLatestReplier = ThreadLatestReplierSaver.UniqueLatestReplier.FromLatestReplier(latestReplierEntity);
-
-            var isExists = _latestRepliersKeyByUnique.TryGetValue(uniqueLatestReplier, out var existingLatestReplier);
-            if (!isExists) _latestRepliersKeyByUnique[uniqueLatestReplier] = latestReplierEntity;
-            t.parsed.LatestReplier = isExists ? existingLatestReplier : latestReplierEntity;
         });
 }

c#/crawler/src/Tieba/Crawl/Saver/Related/ThreadLatestReplierSaver.cs

@@ -1,3 +1,5 @@
+using Microsoft.EntityFrameworkCore.ChangeTracking;
+
 namespace tbm.Crawler.Tieba.Crawl.Saver.Related;
 
 public class ThreadLatestReplierSaver(
@@ -10,19 +12,38 @@ public class ThreadLatestReplierSaver(
 
     public Action SaveFromThread(CrawlerDbContext db, IReadOnlyCollection<ThreadPost> threads)
     {
-        var uniqueLatestRepliers = threads
+        static void DetachThenReplace(
+            EntityEntry<LatestReplier> entityEntry,
+            ThreadPost thread,
+            LatestReplier newLatestReplier)
+        {
+            entityEntry.State = EntityState.Detached;
+            thread.LatestReplier = newLatestReplier;
+        }
+
+        var threadsGroupByUniqueLatestReplier = threads
             .Where(th => th.LatestReplier != null)
-            .Select(UniqueLatestReplier.FromThread).ToList();
+            .GroupBy(UniqueLatestReplier.FromThread).ToList();
+        threadsGroupByUniqueLatestReplier.ForEach(g =>
+            (from thread in g.Skip(1)
+                join entityEntry in db.ChangeTracker.Entries<LatestReplier>()
+                    on thread.LatestReplier equals entityEntry.Entity
+                select (thread, entityEntry))
+            .ForEach(t => DetachThenReplace(t.entityEntry, t.thread, g.First().LatestReplier!)));
+
+        var uniqueLatestRepliers = threadsGroupByUniqueLatestReplier.Select(g => g.Key).ToList();
         var existingLatestRepliers = db.LatestRepliers.AsNoTracking().FilterByItems(
-            uniqueLatestRepliers, (latestReplier, uniqueLatestReplier) =>
-                latestReplier.Name == uniqueLatestReplier.Name
-                && latestReplier.DisplayName == uniqueLatestReplier.DisplayName)
+                uniqueLatestRepliers, (latestReplier, uniqueLatestReplier) =>
+                    latestReplier.Name == uniqueLatestReplier.Name
+                    && latestReplier.DisplayName == uniqueLatestReplier.DisplayName)
             .ToList();
         (from existing in existingLatestRepliers
                 join thread in threads
                     on UniqueLatestReplier.FromLatestReplier(existing) equals UniqueLatestReplier.FromThread(thread)
-                select (existing, thread))
-            .ForEach(t => t.thread.LatestReplier = t.existing);
+                join entityEntry in db.ChangeTracker.Entries<LatestReplier>()
+                    on thread.LatestReplier equals entityEntry.Entity // Object.ReferenceEquals()
+                select (existing, thread, entityEntry))
+            .ForEach(t => DetachThenReplace(t.entityEntry, t.thread, t.existing));
 
         _ = _saverLocks.Value.Acquire(uniqueLatestRepliers
             .Except(existingLatestRepliers.Select(UniqueLatestReplier.FromLatestReplier))