Question

我试图在linq中使用聚合方法到Sql，但看起来它不受支持。

LibrarySystemDataContext ctx = new LibrarySystemDataContext();

var query = ctx.Books
  .Select(x => new
      {
        BookID = x.BookID,
          BookName = x.Title,
          Authors = x.AuthorBooks.Select(a => a.Author)
          .Aggregate(new StringBuilder(), (sb, r) => sb.Append(r.FirstName + " " + r.LastName +"& "), sb => sb.ToString())
          });

使用Aggregate，我立即将书籍转换为IEnumerable，如下所示：

var query = ctx.Books.AsEnumerable()
  .Select(x => new
      {
        BookID = x.BookID,
          BookName = x.Title,
          Authors = x.AuthorBooks.Select(a => a.Author)
          .Aggregate(new StringBuilder(), (sb, r) => sb.Append(r.FirstName + " " + r.LastName +"& "), sb => sb.ToString())
          });

而且，这是有效的。我想知道如果我写这样的查询会有什么缺点或设计缺陷。

Answer 1

使用ctx.Books.AsEnumerable()，您将从数据库中选择整本书籍表并开始在内存中处理它。这不是好设计。您应该做的是，选择您需要的数据，然后在本地处理它。

例如

var temp = ctx.Books
  // probably some where clause
  .Select(x => new
      {
        BookID = x.BookID,
          BookName = x.Title,
          Authors = x.AuthorBooks.Select(a => a.Author)
          })
  .ToArray();

var books = temp.Select(x => new { 
    x.BookID, 
    x.BookName, 
    Authors = x.Authors.Aggregate(new StringBuilder(), (sb, r) => sb.Append(r.FirstName + " " + r.LastName +"& "), sb => sb.ToString()) 
});

不支持聚合扩展方法（Linq to Sql）

1 个答案: