linq - NHibernate Linq 提供者和 take() skip() 与急切获取

Question

我在 NHibernate 3.2 和 SQLite 提供程序中有这个简单的 linq 查询:

var all = (from book in Session.Query<Book>() select book)
    .Skip(15)
    .Take(15)                
    .ToList();

此查询正确返回 15 个实体，但是当我尝试像这样使用 FetchMany 急切加载依赖集合时:

var all = (from book in Session.Query<Book>() select book)
    .FetchMany(books => books.Authors)
    .Skip(15)
    .Take(15)                
    .ToList();

我只返回了 11 个实体。这是错误还是我缺少的东西？

这是生成的 SQL 查询

select
     book0_.Id as Id2_0_,
     author2_.Id as Id0_1_,
     book0_.Title as Title2_0_,
     book0_.Sort as Sort2_0_,
     book0_.TimeStamp as TimeStamp2_0_,
     book0_.PubDate as PubDate2_0_,
     book0_.Series_Index as Series6_2_0_,
     book0_.Author_Sort as Author7_2_0_,
     book0_.Isbn as Isbn2_0_,
     book0_.Lccn as Lccn2_0_,
     book0_.Path as Path2_0_,
     book0_.Flags as Flags2_0_,
     book0_.Uuid as Uuid2_0_,
     book0_.Has_Cover as Has13_2_0_,
     book0_.Last_Modified as Last14_2_0_,
     author2_.Name as Name0_1_,
     author2_.Sort as Sort0_1_,
     author2_.Link as Link0_1_,
     authors1_.book as book0__,
     authors1_.author as author0__
from 
    books book0_ 
  left outer join 
    books_authors_link authors1_ on book0_.Id=authors1_.book left outer join authors author2_ 
  on authors1_.author=author2_.Id
order by book0_.Id asc
limit 15 /* @p0 */ offset 0 /* @p1 */

这有效地将结果集限制为 15 行，而不是我想要的 15 个实体。

Answer 1

skip 和 take 被转换为 sql 中的等价物，它限制了行，并且由于您急切地使用连接进行提取，因此您无能为力。

要急切地获取前 15 本书，您需要:

var all = Session.Query<Book>()
    .Skip(15)
    .Take(15)                
    .ToList();

var ids = all.Select(b => b.Id).ToList();

// fetch all Authors of the books, now the books in all have initialized Authors
Session.Query<Book>()
    .Where(b => ids.Contains(b.Id))
    .FetchMany(books => books.Authors)
    .List();

虽然这有 2 次往返

更新:用QueryOver 来回一次，也许你可以翻译成Linq

var subquery = QueryOver.Of<Book>()
    .Skip(15)
    .Take(15)
    .Select(b => b.Id);

var all = Session.QueryOver<Book>()
    .WithSubquery.WhereProperty(b => b.Id).In(subquery)
    .Fetch(books => books.Authors).Eager
    .ToList();