DbContext对于标量系统函数(Count，Any，Sum，Max)获取IQueryable [英] DbContext get IQueryable for scalar system functions (Count, Any, Sum, Max)

问题描述

我有一个带有DbSet的DBContext，称为Assignments. 创建可枚举表达式的查询并将其连接起来不是问题，但是我看不到通过延迟执行来获得IQueryable的方法，例如Count，Any，Max，Sum.

I have DBContext with DbSet called Assignments. It's not a problem to create queryable for enumerable expressions and concatenated them, however I don't see the way to get IQueryable with deferred execution for functions like Count, Any, Max, Sum.

基本上我想拥有一些IQueryable扩展名，以便可以像这样执行它:

Basically I want to have some IQueryable extension so I can execute it like this:

IQueryable<int> query = 
          myDbContext.SelectValue((ctx)=>ctx.Assignments.Where(...).Count())
.UnionAll(myDbContext.SelectValue((ctx)=>ctx.Assignments.Where(...).Count()));

并获取以下SQL(query.ToString()):

and get the following SQL (query.ToString()):

SELECT 
[UnionAll1].[C1] AS [C1]
FROM  (SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT([Extent1].[UserId]) AS [A1]
        FROM [dbo].[Assignments] AS [Extent1]
                WHERE ...
    )  AS [GroupBy1]
UNION ALL
    SELECT 
    [GroupBy2].[A1] AS [C1]
    FROM ( SELECT 
        COUNT([Extent2].[UserId]) AS [A1]
        FROM [dbo].[Assignments] AS [Extent2]
                WHERE ...
    )  AS [GroupBy2]) AS [UnionAll1]

重要提示:如您所见，我需要能够在带有联合和联接的子查询中使用它，并在最后生成一个SQL REQUEST.我不能使用RAW SQL，也不能为实体使用字符串名称，这就是为什么我看不到ObjectContextAdapter.ObjectContext.CreateQuery对我有用的原因.

IMPORTANT: As you see I need to be able to use it in sub queries, with unions and joins, having ONE SQL REQUEST GENERATED at the end. I cannot use RAW SQL and I cannot use string names for entities, that's why I don't see ObjectContextAdapter.ObjectContext.CreateQuery working for me.

此处 一种使用ObjectContext实现它的方法，但是我不能在我的情况下使用这种方法，因为它会引发错误:

Here you can find a way to achieve it using ObjectContext, but I cannot use this approach for my case, because it throws error:

无法创建赋值"类型的常量值.只有原始的 上下文中支持类型或枚举类型.

Unable to create a constant value of type 'Assignment'. Only primitive types or enumeration types are supported in this context.

推荐答案

与我对其他问题的回答相同的方法在这里也适用.这是使用EF5的独立测试程序:

The same approach as in my answer to that other question works here too. Here is a self-contained test program using EF5:

using System;
using System.Data.Entity;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace ScratchProject
{
    public class A
    {
        public int Id { get; set; }

        public string TextA { get; set; }
    }

    public class B
    {
        public int Id { get; set; }

        public string TextB { get; set; }
    }

    public class MyContext : DbContext
    {
        public DbSet<A> As { get; set; }

        public DbSet<B> Bs { get; set; }

        protected IQueryProvider QueryProvider
        {
            get
            {
                IQueryable queryable = As;
                return queryable.Provider;
            }
        }

        public IQueryable<TResult> CreateScalarQuery<TResult>(Expression<Func<TResult>> expression)
        {
            return QueryProvider.CreateQuery<TResult>(
                Expression.Call(
                    method: GetMethodInfo(() => Queryable.Select<int, TResult>(null, (Expression<Func<int, TResult>>)null)),
                    arg0: Expression.Call(
                        method: GetMethodInfo(() => Queryable.AsQueryable<int>(null)),
                        arg0: Expression.NewArrayInit(typeof(int), Expression.Constant(1))),
                    arg1: Expression.Lambda(body: expression.Body, parameters: new[] { Expression.Parameter(typeof(int)) })));
        }

        static MethodInfo GetMethodInfo(Expression<Action> expression)
        {
            return ((MethodCallExpression)expression.Body).Method;
        }
    }

    static class Program
    {
        static void Main()
        {
            using (var context = new MyContext())
            {
                Console.WriteLine(context.CreateScalarQuery(() => context.As.Count(a => a.TextA != "A"))
                    .Concat(context.CreateScalarQuery(() => context.Bs.Count(b => b.TextB != "B"))));
            }
        }
    }
}

输出:

SELECT
[UnionAll1].[C1] AS [C1]
FROM  (SELECT
        [GroupBy1].[A1] AS [C1]
        FROM ( SELECT
                COUNT(1) AS [A1]
                FROM [dbo].[A] AS [Extent1]
                WHERE N'A' <> [Extent1].[TextA]
        )  AS [GroupBy1]
UNION ALL
        SELECT
        [GroupBy2].[A1] AS [C1]
        FROM ( SELECT
                COUNT(1) AS [A1]
                FROM [dbo].[B] AS [Extent2]
                WHERE N'B' <> [Extent2].[TextB]
        )  AS [GroupBy2]) AS [UnionAll1]

是的，实际上执行查询也可以按预期工作.

And yes, actually executing the query works as expected too.

更新:

根据要求，您可以添加以下内容以使其也适用于Expression<Func<MyContext, TResult>> expression):

As requested, here is what you can add to get it working for Expression<Func<MyContext, TResult>> expression) as well:

public IQueryable<TResult> CreateScalarQuery<TResult>(Expression<Func<MyContext, TResult>> expression)
{
    var parameterReplacer = new ParameterReplacer(expression.Parameters[0], Expression.Property(Expression.Constant(new Tuple<MyContext>(this)), "Item1"));
    return CreateScalarQuery(Expression.Lambda<Func<TResult>>(parameterReplacer.Visit(expression.Body)));
}

class ParameterReplacer : ExpressionVisitor
{
    readonly ParameterExpression parameter;
    readonly Expression replacement;

    public ParameterReplacer(ParameterExpression parameter, Expression replacement)
    {
        this.parameter = parameter;
        this.replacement = replacement;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node == parameter)
            return replacement;

        return base.VisitParameter(node);
    }
}

即使从当前上下文内部调用也可以:

This works even if called from inside the current context:

// member of MyContext
public void Test1()
{
    Console.WriteLine(this.CreateScalarQuery(ctx => ctx.As.Count(a => a.TextA != "A"))
        .Concat(this.CreateScalarQuery(ctx => ctx.Bs.Count(b => b.TextB != "B"))));
}

参数替换将上下文直接存储在Tuple<MyContext>中而不是MyContext中，因为EF不知道如何处理Expression.Constant(this).那是C#编译器永远不会产生的东西，因此EF不需要知道如何处理它.将上下文作为类的成员获得是C#编译器确实产生的东西，因此使EF知道如何处理它.

The parameter replacement stores the context in a Tuple<MyContext> instead of MyContext directly, because EF does not know how to handle Expression.Constant(this). That's something that the C# compiler will never produce anyway, so EF does not need to know how to handle it. Getting a context as a member of a class is something that the C# compiler does produce, so EF has been made to know how to handle that.

但是，如果将this保存在本地变量中，则可以使CreateScalarQuery的更简单版本起作用:

However, the simpler version of CreateScalarQuery can be made to work too, if you save this in a local variable:

// member of MyContext
public void Test2()
{
    var context = this;
    Console.WriteLine(this.CreateScalarQuery(() => context.As.Count(a => a.TextA != "A"))
        .Concat(this.CreateScalarQuery(() => context.Bs.Count(b => b.TextB != "B"))));
}

这篇关于DbContext对于标量系统函数(Count，Any，Sum，Max)获取IQueryable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！