perf: reduce topic list queries

backend/app/services/content.go

@@ -539,32 +539,85 @@ func (s *content) ListTopics(ctx context.Context, tenantID int64) ([]content_dto
 		return nil, errorx.ErrDatabaseError.WithCause(err)
 	}
 
-	var topics []content_dto.Topic
+	if len(results) == 0 {
+		return []content_dto.Topic{}, nil
+	}
+
+	genres := make([]string, 0, len(results))
+	for _, r := range results {
+		if r.Genre == "" {
+			continue
+		}
+		genres = append(genres, r.Genre)
+	}
+
+	contentIDByGenre := make(map[string]int64, len(genres))
+	contentMap := make(map[int64]*models.Content, len(genres))
+	if len(genres) > 0 {
+		// 批量获取每个分类最新内容，避免逐条查询导致 N+1。
+		var rows []struct {
+			Genre     string `gorm:"column:genre"`
+			ContentID int64  `gorm:"column:content_id"`
+		}
+		contentQuery := models.ContentQuery.WithContext(ctx).UnderlyingDB().
+			Model(&models.Content{}).
+			Select("distinct on (genre) genre, id as content_id").
+			Where("status = ?", consts.ContentStatusPublished).
+			Where("genre IN ?", genres).
+			Order("genre, published_at desc, id desc")
+		if tenantID > 0 {
+			contentQuery = contentQuery.Where("tenant_id = ?", tenantID)
+		}
+		if err := contentQuery.Scan(&rows).Error; err != nil {
+			return nil, errorx.ErrDatabaseError.WithCause(err)
+		}
+
+		contentIDs := make([]int64, 0, len(rows))
+		seen := make(map[int64]struct{}, len(rows))
+		for _, row := range rows {
+			if row.ContentID == 0 {
+				continue
+			}
+			contentIDByGenre[row.Genre] = row.ContentID
+			if _, ok := seen[row.ContentID]; ok {
+				continue
+			}
+			seen[row.ContentID] = struct{}{}
+			contentIDs = append(contentIDs, row.ContentID)
+		}
+
+		if len(contentIDs) > 0 {
+			var contents []*models.Content
+			if err := models.ContentQuery.WithContext(ctx).
+				UnderlyingDB().
+				Preload("ContentAssets").
+				Preload("ContentAssets.Asset").
+				Where("id IN ?", contentIDs).
+				Find(&contents).Error; err != nil {
+				return nil, errorx.ErrDatabaseError.WithCause(err)
+			}
+			for _, c := range contents {
+				contentMap[c.ID] = c
+			}
+		}
+	}
+
+	topics := make([]content_dto.Topic, 0, len(results))
 	for i, r := range results {
 		if r.Genre == "" {
 			continue
 		}
 
-		// Fetch latest content in this genre to get a cover
-		var c models.Content
-		query := models.ContentQuery.WithContext(ctx).
-			Where(models.ContentQuery.Genre.Eq(r.Genre), models.ContentQuery.Status.Eq(consts.ContentStatusPublished))
-		if tenantID > 0 {
-			query = query.Where(models.ContentQuery.TenantID.Eq(tenantID))
-		}
-		query.
-			Order(models.ContentQuery.PublishedAt.Desc()).
-			UnderlyingDB().
-			Preload("ContentAssets").
-			Preload("ContentAssets.Asset").
-			First(&c)
-
 		cover := ""
-		for _, ca := range c.ContentAssets {
-			if (ca.Role == consts.ContentAssetRoleCover || ca.Role == consts.ContentAssetRoleMain) && ca.Asset != nil {
-				cover = Common.GetAssetURL(ca.Asset.ObjectKey)
-				if ca.Role == consts.ContentAssetRoleCover {
-					break // Prefer cover
+		if contentID, ok := contentIDByGenre[r.Genre]; ok {
+			if c := contentMap[contentID]; c != nil {
+				for _, ca := range c.ContentAssets {
+					if (ca.Role == consts.ContentAssetRoleCover || ca.Role == consts.ContentAssetRoleMain) && ca.Asset != nil {
+						cover = Common.GetAssetURL(ca.Asset.ObjectKey)
+						if ca.Role == consts.ContentAssetRoleCover {
+							break // Prefer cover
+						}
+					}
 				}
 			}
 		}