包括衍生财产成的LINQ to实体查询 [英] Include derived property into a linq to entity query

问题描述

下面是一个基于名为任务 A波科类的简单项目：

Here is a simple project based on a Poco class named Task:

class Program
{
    static void Main(string[] args)
    {
        using (MyDbContext ctx = new MyDbContext())
        {
            // first query
            DateTime compareDate = DateTime.Now + TimeSpan.FromDays(3);
            var res = ctx.Tasks.Where(t => t.LastUpdate < compareDate).ToList();

            // second query
            res = ctx.Tasks.Where(t => t.ShouldUpdate).ToList();
        }
    }
}

public class MyDbContext : DbContext
{
    public DbSet<Task> Tasks { get; set; }
}

public class Task
{
    public int ID { get; set; }
    public DateTime LastUpdate { get; set; }
    public  bool ShouldUpdate
    {
        get
        {
            return LastUpdate < DateTime.Now + TimeSpan.FromDays(3);
        }
    }
}

我想要做的就是查询上下文 dbset 包括在where子句中的 ShouldUpdate 衍生属性。

What I want to do is to query the context dbset including in the where clause the ShouldUpdate derived property.

第一查询工作正常（我不能写在一行，但它并不重要）。

The "first query works fine" (I can't write it in a single line but it doesn't matter).

正如你所知道，我们得到了一个引发NotSupportedException 关于十二查询，以下消息：在LINQ是不支持的实体
指定的类型成员ShouldUpdate。只有初始化，实体成员和实体导航属性都支持。

As you know, we get a NotSupportedException on the "second query", with the following message: The specified type member 'ShouldUpdate' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.

这是正确的，我可以理解为什么它会发生，但我需要封装在<$派生信息C $ C>工作对象，这样我就可以在网格中显示的属性或使用它在所有其他地方，而不用复制它背后的逻辑。

That's right and I can understand why it happen, but I need to encapsulate the derived information inside the Task object so I can display the property in a grid or use it in every other place, without duplicating the logic behind it.

有一个聪明的方法来做到这一点？

Is there a smart technique to do this?

注：什么是 ShouldUplate 属性的技术名称？衍生？如何计算？计算的？

NB: What is the technical name of the ShouldUplate property? derived? calculated? computed?

推荐答案

最后我找到了解决办法..
你可以存储静态个行业部分查询（表达式）并利用它们是这样的：

Finally i found THE solution.. You can store the partial queries (Expressions) in static fileds and use them like this:

class Program
{
    static void Main(string[] args)
    {
        using (MyDbContext ctx = new MyDbContext())
        {
            res = ctx.Tasks.Where(Task.ShouldUpdateExpression).ToList();
        }
    }
}

public class MyDbContext : DbContext
{
    public DbSet<Task> Tasks { get; set; }
}

public class Task
{
    public int ID { get; set; }
    public DateTime LastUpdate { get; set; }
    public bool ShouldUpdate
    {
        get
        {
            return ShouldUpdateExpression.Compile()(this);
        }
    }

    public static Expression<Func<Task, bool>> ShouldUpdateExpression
    {
        get
        {
            return t => t.LastUpdate < EntityFunctions.AddDays(DateTime.Now, 3);
        }
    }
}

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