有条件地将儿童财产排除在实体框架查询中

Question

给定以下对象图

public class Bar {
    public Guid Id {get;set;}
    public string SomeBar {get;set;}
}
public class Foo {
    public Guid Id {get;set;}
    public string FooString {get; set;}
    public IList<Bar> Bars {get;set;}
}

如何正确排除Bars属性，以使实体框架不会加载整个对象图？

// my thoughts
public Task<Foo> GetFooByIdAsync(Guid id, bool includeBars) 
{
    var query = _db.Foos.Where(f => f.Id == id);
    if(!includeBars)
    {
        return query.Select(f=> new Foo{
            Id = f.Id,
            FooString = f.FooString
        }).ToListAsync();
    } else {
        return query.ToListAsync();
    }
}

Answer 1

只需有条件地将Include（f => f.bars）添加到查询表达式中即可。这样：

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Common;
using System.Data.Entity;
using System.Data.Entity.ModelConfiguration;
using System.Data.SqlClient;
using System.Linq;
using System.Threading.Tasks;

namespace ConsoleApp8
{
    public class Bar
    {
        public Guid Id { get; set; }
        public string SomeBar { get; set; }
    }
    public class Foo
    {
        public Guid Id { get; set; }
        public string FooString { get; set; }
        public IList<Bar> Bars { get; set; }
    }
    class Db: DbContext
    {
        public DbSet<Foo> Foos { get; set; }
        public DbSet<Bar> Bar { get; set; }
        public async Task<Foo> GetFooByIdAsync(Guid id, bool includeBars)
        {
            var query = (IQueryable<Foo>)Foos.AsNoTracking();
            if (includeBars)
            {
                query = query.Include(f => f.Bars);
            }
            query = query.Where(f => f.Id == id);
            var results = await query.ToListAsync();
            return results.FirstOrDefault();            
        }
    }

    class Program
    {      
        static void Main(string[] args)
        {
            Database.SetInitializer(new DropCreateDatabaseAlways<Db>());

            using (var db = new Db())
            {
                db.Database.Initialize(false);
                db.Database.Log = m => Console.WriteLine(m);
                var r1 = db.GetFooByIdAsync(Guid.Empty, false).Result;
                var r2 = db.GetFooByIdAsync(Guid.Empty, true).Result;
            }
            Console.WriteLine("Hit any key to exit");
            Console.ReadKey();

        }
    }
}

输出

Opened connection asynchronously at 9/11/2017 11:37:26 PM -05:00
SELECT
    [Extent1].[Id] AS [Id],
    [Extent1].[FooString] AS [FooString]
    FROM [dbo].[Foos] AS [Extent1]
    WHERE [Extent1].[Id] = @p__linq__0

-- p__linq__0: '00000000-0000-0000-0000-000000000000' (Type = Guid, IsNullable = false)
-- Executing asynchronously at 9/11/2017 11:37:26 PM -05:00
-- Completed in 22 ms with result: SqlDataReader

Closed connection at 9/11/2017 11:37:26 PM -05:00
Opened connection asynchronously at 9/11/2017 11:37:26 PM -05:00
SELECT
    [Project1].[C1] AS [C1],
    [Project1].[Id] AS [Id],
    [Project1].[FooString] AS [FooString],
    [Project1].[C2] AS [C2],
    [Project1].[Id1] AS [Id1],
    [Project1].[SomeBar] AS [SomeBar]
    FROM ( SELECT
        [Extent1].[Id] AS [Id],
        [Extent1].[FooString] AS [FooString],
        1 AS [C1],
        [Extent2].[Id] AS [Id1],
        [Extent2].[SomeBar] AS [SomeBar],
        CASE WHEN ([Extent2].[Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
        FROM  [dbo].[Foos] AS [Extent1]
        LEFT OUTER JOIN [dbo].[Bars] AS [Extent2] ON [Extent1].[Id] = [Extent2].[Foo_Id]
        WHERE [Extent1].[Id] = @p__linq__0
    )  AS [Project1]
    ORDER BY [Project1].[Id] ASC, [Project1].[C2] ASC

-- p__linq__0: '00000000-0000-0000-0000-000000000000' (Type = Guid, IsNullable = false)
-- Executing asynchronously at 9/11/2017 11:37:26 PM -05:00
-- Completed in 2 ms with result: SqlDataReader

Closed connection at 9/11/2017 11:37:26 PM -05:00
Hit any key to exit