展平IQueryable< T>时发生错误在GroupBy()之后 [英] Error while flattening the IQueryable<T> after GroupBy()

问题描述

我正在尝试从数据库中获取扁平化的对象，但出现错误.我想获得数据库中所有流派的2本书籍，代码如下:

I am trying to get flattened object from the database, but I get an error. I want to get 2 books for all genres in the database, the code looks like this:

IQueryable<Book> query = _context.Books
                                 .GroupBy(b => b.Genre)
                                 .SelectMany(bc => bc.Select(b => b).Take(2));

有人知道我在这里做什么错吗?

Does anyone know what am I doing wrong in here?

我得到这个异常而不是结果:

I get this exception instead of a result :

LINQ表达式' bc =>bc .AsQueryable().Select(b => b).Take(2)'无法翻译.以一种可以翻译的形式重写查询，或者通过插入对"AsEnumerable"，"AsAsyncEnumerable"，"ToList"或"ToListAsync"的调用来显式切换到客户端评估.有关更多信息，请参见go.microsoft.com/fwlink/?linkid=2101038.

The LINQ expression 'bc => bc .AsQueryable() .Select(b => b) .Take(2)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See go.microsoft.com/fwlink/?linkid=2101038 for more information.

我也尝试过类似的事情:

I tried also something like that :

IQueryable<Book> query = _context.Genres.GroupJoin(
     _context.Books,
     g => g,
     b => b.Genre,
     (g, books) => new
     {
         Genre = g,
         BookCollection = books
     }
    ).SelectMany(bc => bc.BookCollection.Select(b => b)
    .Take(2)).Include(b => b.Author).Include(b => b.Rating)
             .Include(b => b.BookISBNs).Include(b => b.Reviews)
             .ThenInclude(r => r.User);

推荐答案

此尝试偏离了分组依据，这在数据库和EF查询转换中是有限制的.取而代之的是，我们要去一个子查询.

This attempt moves away from group by, which is restrictive in the database and EF query translation. Instead, we're going for a subquery.

var genres = _context.Books.Select(b => b.Genre).Distinct();
var books =
  from genre in genres
  from book in _context.Books.Where(b => b.Genre == genre).Take(2)
  select book;

var results = books.ToArrayAsync();

针对这种翻译:

SELECT c.*
FROM
  (SELECT DISTINCT Genre FROM Books) as a,
  (SELECT top 2 b.* FROM Books b WHERE b.Genre = a.Genre) as c

这篇关于展平IQueryable< T>时发生错误在GroupBy()之后的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！