订单实体框架通过rowversion属性查询，在存储和内存中 [英] Order Entity Framework query by rowversion property, in store and in memory

问题描述

假设我的（Code First）实体框架模型中有一个实体，如下所示：

  public class Foo 
 {
 [Key] 
 [MaxLength（50）] 
 public string FooId {get;组; 
 
 [Timestamp] 
 [ConcurrencyCheck] 
 public byte [] Version {get;组; } 
} 
  

进一步假设我需要获取最近几个这些项目。我可以用一个相当简单的LINQ表达式来实现，它被转换成正确的SQL，并且运行正常。

  var recentFoos = db.Foos.OrderBy（f => f.Version）.Take（10）; 
  

现在，如果这个查询发生在我想要测试的方法中，事情会变得更加复杂。我有一个假的上下文和实体集合的设置，一切都正常。但是，因为 byte [] 不能直接排序，尝试执行该代码会抛出一个 ArgumentException （At至少一个对象必须实现IComparable。）。

另一方面，如果我使用 OrderBy（）它采用比较器，这将在内存中运行，但会抛出异常（...此方法无法转换为存储表达式）尝试针对数据库运行。

有没有办法a）编写一个可以在两种情况下正确运行的LINQ查询，或者b）可靠地检测哪种情况，以便我的代码可以使用正确的重载进行排序？ / strong>

解决方案

我最终实施了解决方案 jnm2建议：写一个 ExpressionVisitor 重写查询从正常查询使用的谓词唯一重载到内存查询所必需的重载。我不得不修改原始Microsoft示例代码为假的 DbSet 允许这些访问者在正确的时刻插入并使用。

一个Gist与我的代码可以在这里找到。如果有人可以提出更优雅的方式来申请访问者，我绝对愿意提供建议。

Assume I have an entity in my (Code First) Entity Framework model that looks like this:

public class Foo
{
  [Key]
  [MaxLength(50)]
  public string FooId { get; set; }

  [Timestamp]
  [ConcurrencyCheck]
  public byte[] Version { get; set; }
}

Assume further that I need to fetch the most recent few of these items. I can do that with a fairly simple LINQ expression, which gets translated into the correct SQL, and runs fine.

var recentFoos = db.Foos.OrderBy(f => f.Version).Take(10);

Now, if that query happens in a method that I also want to test, things get more complicated. I have a setup with fake context and entity collections, and that all works fine. But, because byte[] can't be sorted directly, trying to execute that code throws an ArgumentException ("At least one object must implement IComparable.").

On the other hand, if I use the overload of OrderBy() that take a comparer, that will run in memory, but throw an exception ("…this method cannot be translated into a store expression.") trying to run against the database.

Is there a way to either a) write a single LINQ query that will run correctly in both circumstances, or b) to reliably detect which case pertains, so that my code can sort using the correct overload?

解决方案

I ended up implementing the solution jnm2 suggested: writing an ExpressionVisitor to rewrite the query from the predicate-only overload used by normal queries to the overload necessary for in-memory querying. I had to rework the original Microsoft sample code for fake DbSet to allow these visitors to be plugged in and used at the right moment.

A Gist with my code can be found here. If anyone can suggest a more elegant way to apply the visitors, I'm definitely open to suggestions.

这篇关于订单实体框架通过rowversion属性查询，在存储和内存中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！