实体框架执行表达式

Question

以下是简化后的代码

from oi in orderItems
    group oiGrouped by ...
    into orderItemsGroupedBySomething
    select new
    {
        Key = orderItemsGroupedBySomething.Key,
        Revenue = /*Here is some code that I want to extract to separate method, for example*/ 
            orderItemsGroupedBySomething.Sum(x => x.UnitPrice * x.Quantity)                     
    }

实际上我的情况更复杂。但我认为这无关紧要。我无法提取到orderItemsGroupedBySomething.Sum(x => x.UnitPrice * x.Quantity)的简单方法计算，因为它不是EntityFramework的已知方法。我试着把它的表达式，但我得到错误"The LINQ expression node type 'Invoke' is not supported in LINQ to Entities."我编译表达式之前使用它的查询，我认为因此我得到错误。我该如何解决这个问题?

Answer 1

我不知道你需要它有多通用，但像这样的东西应该工作:

void Main()
{
    OrderItems.GroupBy(oi => oi.SomeProp).Select(GetExpression());
}
public Expression<Func<IGrouping<KeyType, OrderItem>, dynamic>> GetExpression()
{
    return (ig) => new { Key = ig.Key, Revenue = ig.Sum(x => x.UnitPrice * x.Quantity) };
}

edit:在分组的情况下，我可能会返回Tuple而不是匿名类型。

Answer 2

我添加了返回表达式的方法

public Expression<Func<OrderItem, decimal>> GetExpression()
{
    return x => x.UnitPrice*x.Quantity;
}

Then I try

from oi in orderItems
    group oiGrouped by ...
    into orderItemsGroupedBySomething
    select new
    {
        Key = orderItemsGroupedBySomething.Key,
        Revenue =  orderItemsGroupedBySomething.Sum(GetExpression())                     
    }

但是它不起作用，比如@LorentzVedeler的答案。因为orderItemsGroupedBySomething是igrouing类型的，没有Sum方法和表达式类型的参数。因此我尝试了

orderItemsGroupedBySomething.AsQueryable.Sum(GetExpression())

引起Internal .NET Framework Data Provider error 1025。问题是我在linq中调用了GetExpression()方法。为了解决这个问题，我把表达式放到局部变量中。结果

var expression = GetExpression();
from oi in orderItems
        group oiGrouped by ...
        into orderItemsGroupedBySomething
        select new
        {
            Key = orderItemsGroupedBySomething.Key,
            Revenue =  orderItemsGroupedBySomething.AsQueryable.Sum(expression)                     
        }