在没有 N+1 的情况下通过 NHibernate 进行查询 - 包含示例

Question

我有一个 N+1 问题，我不知道如何解决它。

可以在这个问题的底部找到一个完全可重现的样本。所以如果你愿意，请创建数据库，设置 NUnit 测试和所有伴随的 classes，并尝试在本地消除 N+1。这是我遇到的一个真实问题的匿名版本。 众所周知，此代码对于帮助发射下一个 space 月球航天飞机至关重要。 如果有人问我，我不会否认。

总结问题：我正在尝试查询下面绘制的 table 结构。关于这个 table 结构，唯一需要注意的奇怪之处是问题有选择，然后有子问题，然后有子选择。你只能假设2个级别的问题->选择->问题->选择。

SiteSurveyQuestion
|
+---Site
|
+---Survey
|
+---Question
    |
    +---Choice
    +---Choice
    +---Choice
        |
        +---Question
        +---Question
        +---Question
            |
            +---Choice
            +---Choice
            +---Choice

我已经尽我所能去尝试。

在映射中，我尝试了一堆引用字段 .Not.LazyLoad() 但没有真正成功。

我还尝试通过添加 .Fetch() 和 .FetchMany() 以及 .ThenFetchMany() 的许多组合来修改查询，甚至尝试运行多个 .ToFuture() 查询.这些 do 对 SQL 查询进行了真正的更改，但不是我正在寻找的最终结果。

查询归结为 "get me a list of all questions for this survey on this site, including all sub-questions"。这是查询：

using (var session = sessionFactory.OpenSession())
{
    var questionsForSurvey = session.Query<SiteSurveyQuestion>()
        .Where(x => x.Site.Id == 1 && x.Survey.Id == 1)
        .ToArray();
}

所以最后问这个问题：我该如何解决这个 N+1 问题？我会对以下任何一项感到满意

（首选）class 映射中的修复以预先加载所有内容
（第二选择）使用 LINQ 提供程序在查询中添加提取或查询提示
（第三选择）以上的混合
（第 4 个选择）被告知这是不可能的，并且是 NHibernate 的限制
（第 5 次选择）HQL 中的解决方案

我不想要 HQL 解决方案，因为我不会了解我在映射 and/or 查询中做错了什么 - 我觉得我缺少一些基本的东西，我什至不知道去哪里看。

示例说明：

将 SQL 安装脚本复制并粘贴到本地 SQL 服务器实例中，运行它。
创建一个测试项目（或者如果你很懒，使用现有的测试项目）并将 NHibernate 和 Fluent NHibernate 的 nuget 包添加到项目中。
运行测试。你应该看到：
1. 由 NHibernate
2. 测试的输出。
修复 mappings/query 直到 N+1 消失 - 你会知道当你首先看到一堆 SQL 脚本输出时，然后：

```

Site: Site1 Survey: SurveyAboutCats Q: Own A Cat?
    o Yes
        Q: How many cats did you feed yesterday?
            o 1
            o 2-5
            o 6-10
            o 11-20
            o 20+
            o 100+
        Q: How much do you spend on cats annually?
            o 0-100
            o 100-500
            o 500-2000
            o 2000+
    o No
        Q: No cats? What is wrong with you?
            o I am sorry
Site: Site1 Survey: SurveyAboutCats Q: Own A Dog?
    o Yes
    o No

完整样本：

/*
Nuget packages:

  <package id="FluentNHibernate" version="1.3.0.733" targetFramework="net40" />
  <package id="NHibernate" version="3.3.3.4001" targetFramework="net45" />
  <package id="NUnit" version="2.6.2" targetFramework="net40" />

*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using FluentNHibernate.Cfg;
using FluentNHibernate.Cfg.Db;
using FluentNHibernate.Conventions.Helpers;
using FluentNHibernate.Mapping;
using NHibernate;
using NHibernate.Cfg;
using NHibernate.Linq;
using NUnit.Framework;

namespace Whosebug.CryForHelp
{
    [TestFixture]
    public class NHibernateMappingTests
    {
        [Test]
        public void ShouldMapEntitiesWithoutNPlusOneIssue()
        {
            //Arrange
            var connectionString = "Data Source=(local);Initial Catalog=NinetyNineProblemsAndAnNPlusOne;Integrated Security=SSPI;";

            Configuration applicationConfiguration = new Configuration();
            applicationConfiguration.SetProperty("connection.provider", "NHibernate.Connection.DriverConnectionProvider");
            applicationConfiguration.SetProperty("dialect", "NHibernate.Dialect.MsSql2008Dialect");
            applicationConfiguration.SetProperty("connection.driver_class", "NHibernate.Driver.SqlClientDriver");
            applicationConfiguration.SetProperty("default_schema", "dbo");
            applicationConfiguration.SetProperty("format_sql", "format_sql");
            applicationConfiguration.SetProperty("show_sql", "true");
            applicationConfiguration.SetProperty("generate_statistics", "true");
            applicationConfiguration.Configure();
            Configuration fluentConfiguration = null;

            ISessionFactory sessionFactory = Fluently.Configure(applicationConfiguration)
                .Mappings(m =>
                {
                    m.FluentMappings.Conventions.Setup(x => x.Add(AutoImport.Never()));
                    m.FluentMappings.AddFromAssembly(Assembly.GetAssembly(GetType()));
                })

                .ExposeConfiguration(c => fluentConfiguration = c)
                .Database(MsSqlConfiguration.MsSql2008.ConnectionString(connectionString))
                .BuildSessionFactory();

            var mappings = fluentConfiguration.ClassMappings;

            //Act + Assert that we please don't create N+1 queries
            using (var session = sessionFactory.OpenSession())
            {
                var questionsForSurvey = session.Query<SiteSurveyQuestion>()
                    .Where(x => x.Site.Id == 1 && x.Survey.Id == 1)
                    .ToArray();

                foreach (var question in questionsForSurvey)
                {
                    Console.WriteLine("Site: {0} Survey: {1} Q: {2}", question.Site.Name, question.Survey.Name, question.Question.InternalName);

                    foreach (var choice in question.Question.Choices)
                    {
                        Console.WriteLine("\t> " + choice.InternalName);

                        foreach (var subQuestion in choice.Questions)
                        {
                            Console.WriteLine("\t\tQ: " + subQuestion.InternalName);

                            foreach (var subChoice in subQuestion.Choices)
                                Console.WriteLine("\t\t\t> " + subChoice.InternalName);
                        }
                    }
                }
            }
        }
    }


    public class Site
    {
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
    }

    public class Survey
    {
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
    }

    public class SiteSurvey
    {
        public virtual Site Site { get; set; }
        public virtual Survey Survey { get; set; }
        public virtual string Status { get; set; }
        public virtual string Name { get; set; }

        public virtual bool Equals(SiteSurvey other)
        {
            if (ReferenceEquals(null, other)) return false;
            if (ReferenceEquals(this, other)) return true;
            return Site.Id == other.Site.Id && Survey.Id == other.Survey.Id;
        }

        public override bool Equals(object obj)
        {
            if (ReferenceEquals(null, obj)) return false;
            if (ReferenceEquals(this, obj)) return true;
            if (obj.GetType() != this.GetType()) return false;
            return Equals((SiteSurvey) obj);
        }

        public override int GetHashCode()
        {
            unchecked
            {
                return (Survey.Id * 397) ^ Site.Id;
            }
        }
    }

    public class SiteSurveyQuestion
    {
        public virtual Site Site { get; set; }
        public virtual Survey Survey { get; set; }
        public virtual Question Question { get; set; }
        public virtual bool IsActive { get; set; }

        public virtual bool Equals(SiteSurveyQuestion other)
        {
            if (ReferenceEquals(null, other)) return false;
            if (ReferenceEquals(this, other)) return true;
            return Site.Id == other.Site.Id && Survey.Id == other.Survey.Id && Question.Id == other.Question.Id;
        }

        public override bool Equals(object obj)
        {
            if (ReferenceEquals(null, obj)) return false;
            if (ReferenceEquals(this, obj)) return true;
            if (obj.GetType() != this.GetType()) return false;
            return Equals((SiteSurveyQuestion) obj);
        }

        public override int GetHashCode()
        {
            unchecked
            {
                return Question.Id ^ (((Survey.Id * 397) ^ Site.Id) * 397);
            }
        }
    }

    public class Question
    {
        public virtual int Id { get; set; }
        public virtual string InternalName { get; set; }
        public virtual bool IsActive { get; set; }
        public virtual IEnumerable<Choice> Choices { get; set; }
    }

    public class Choice
    {
        public virtual int Id { get; set; }
        public virtual string InternalName { get; set; }
        public virtual bool IsActive { get; set; }
        public virtual IEnumerable<Question> Questions { get; set; }
    }

    public class SurveyMap : ClassMap<Survey>
    {
        public SurveyMap()
        {
            Table("Surveys");
            Id(x => x.Id, "SurveyId").GeneratedBy.Identity().UnsavedValue(0);
            Map(x => x.Name).Not.Nullable();
        }
    }

    public class SiteMap : ClassMap<Site>
    {
        public SiteMap()
        {
            Table("Sites");
            Id(x => x.Id, "SiteId").GeneratedBy.Identity().UnsavedValue(0);
            Map(x => x.Name, "Name").Not.Nullable();
        }
    }

    public class SiteSurveyMap : ClassMap<SiteSurvey>
    {
        public SiteSurveyMap()
        {
            Table("SiteSurveys");
            CompositeId()
                .KeyReference(x => x.Site, "SiteId")
                .KeyReference(x => x.Survey, "SurveyId");

            Map(x => x.Status).Not.Nullable();
            Map(x => x.Name).Not.Nullable();
        }
    }

    public class SiteSurveyQuestionMap : ClassMap<SiteSurveyQuestion>
    {
        public SiteSurveyQuestionMap()
        {
            Table("SiteSurveyQuestions");
            CompositeId()
                .KeyReference(x => x.Site, "SiteId")
                .KeyReference(x => x.Survey, "SurveyId")
                .KeyReference(x => x.Question, "QuestionId");

            Map(x => x.IsActive, "ActiveFlag").Not.Nullable();
        }
    }

    public class QuestionMap : ClassMap<Question>
    {
        public QuestionMap()
        {
            Table("Questions");
            Id(x => x.Id, "QuestionId").GeneratedBy.Identity().UnsavedValue(0);
            Map(x => x.InternalName);
            Map(x => x.IsActive, "ActiveFlag");

            HasMany(x => x.Choices).KeyColumn("QuestionId").AsBag().Cascade.AllDeleteOrphan().Inverse().Not.LazyLoad();

        }
    }

    public class ChoiceMap : ClassMap<Choice>
    {
        public ChoiceMap()
        {
            Table("Choices");
            Id(x => x.Id, "ChoiceId").GeneratedBy.Identity().UnsavedValue(0);
            Map(x => x.InternalName);
            Map(x => x.IsActive, "ActiveFlag");
            HasMany(x => x.Questions)
                .KeyColumn("ChoiceId")
                .AsBag()
                .Cascade
                .AllDeleteOrphan()
                .Inverse();
        }
    }
}

/*









use [master]
GO

CREATE DATABASE [NinetyNineProblemsAndAnNPlusOne]
GO

USE [NinetyNineProblemsAndAnNPlusOne]
GO

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

SET ANSI_PADDING ON
GO

CREATE TABLE [dbo].[Sites](
    [SiteId] [int] IDENTITY(1,1) NOT NULL,
    [Name] [varchar](100) NOT NULL,
 CONSTRAINT [XPKSites] PRIMARY KEY CLUSTERED 
(
    [SiteId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [XAK1Sites] UNIQUE NONCLUSTERED 
(
    [Name] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

CREATE TABLE [dbo].[Surveys](
    [SurveyId] [int] IDENTITY(1,1) NOT NULL,
    [Name] [varchar](500) NOT NULL,
    [Status] [varchar](12) NOT NULL,
    [SurveyTypeId] [int] NOT NULL,
 CONSTRAINT [XPKSurveys] PRIMARY KEY CLUSTERED 
(
    [SurveyId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [XAK1Surveys] UNIQUE NONCLUSTERED 
(
    [Name] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO


CREATE TABLE [dbo].[SiteSurveys](
    [SiteId] [int] NOT NULL,
    [SurveyId] [int] NOT NULL,
    [Name] [varchar](500) NOT NULL,
    [Status] [varchar](12) NOT NULL,
 CONSTRAINT [XPKSiteSurveys] PRIMARY KEY CLUSTERED 
(
    [SiteId] ASC,
    [SurveyId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [XAK1SiteSurveys] UNIQUE NONCLUSTERED 
(
    [SiteId] ASC,
    [SurveyId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [XAK2SiteSurveys] UNIQUE NONCLUSTERED 
(
    [SiteId] ASC,
    [Name] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

CREATE TABLE [dbo].[SiteSurveyQuestions](
    [SiteId] [int] NOT NULL,
    [SurveyId] [int] NOT NULL,
    [QuestionId] [int] NOT NULL,
    [SurveyQuestionTypeId] [int] NULL,
    [ActiveFlag] [bit] NOT NULL,
    [IsRequired] [bit] NOT NULL,
 CONSTRAINT [XPKSurveyQuestions] PRIMARY KEY CLUSTERED 
(
    [SurveyId] ASC,
    [QuestionId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO


CREATE TABLE [dbo].[Questions](
    [QuestionId] [int] IDENTITY(1,1) NOT NULL,
    [InternalName] [varchar](100) NOT NULL,
    [ChoiceId] [int] NULL,
    [ActiveFlag] [bit] NOT NULL,
 CONSTRAINT [XPKQuestions] PRIMARY KEY CLUSTERED 
(
    [QuestionId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [XAK1QuestionsInternalName] UNIQUE NONCLUSTERED 
(
    [InternalName] ASC,
    [ChoiceId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

CREATE TABLE [dbo].[Choices](
    [ChoiceId] [int] IDENTITY(1,1) NOT NULL,
    [QuestionId] [int] NOT NULL,
    [InternalName] [varchar](100) NOT NULL,
    [ActiveFlag] [bit] NOT NULL,
 CONSTRAINT [XPKChoices] PRIMARY KEY CLUSTERED 
(
    [ChoiceId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [XAKChoiceIdQuestionId] UNIQUE NONCLUSTERED 
(
    [ChoiceId] ASC,
    [QuestionId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [XAKChoiceInternalName] UNIQUE NONCLUSTERED 
(
    [QuestionId] ASC,
    [InternalName] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[Surveys]  WITH CHECK ADD  CONSTRAINT [VRSurveyStatuses_Surveys] CHECK  (([Status]='Live' OR [Status]='NotLive' OR [Status]='Discontinued'))
GO

ALTER TABLE [dbo].[Surveys] CHECK CONSTRAINT [VRSurveyStatuses_Surveys]
GO

ALTER TABLE [dbo].[SiteSurveys]  WITH CHECK ADD  CONSTRAINT [R289] FOREIGN KEY([SurveyId])
REFERENCES [dbo].[Surveys] ([SurveyId])
GO

ALTER TABLE [dbo].[SiteSurveys] CHECK CONSTRAINT [R289]
GO

ALTER TABLE [dbo].[SiteSurveys]  WITH CHECK ADD  CONSTRAINT [R303] FOREIGN KEY([SiteId])
REFERENCES [dbo].[Sites] ([SiteId])
GO

ALTER TABLE [dbo].[SiteSurveys] CHECK CONSTRAINT [R303]
GO

ALTER TABLE [dbo].[SiteSurveys]  WITH CHECK ADD  CONSTRAINT [VRSurveyStatuses_SiteSurveys] CHECK  (([Status]='Live' OR [Status]='NotLive' OR [Status]='Discontinued'))
GO

ALTER TABLE [dbo].[SiteSurveys] CHECK CONSTRAINT [VRSurveyStatuses_SiteSurveys]
GO


ALTER TABLE [dbo].[SiteSurveyQuestions]  WITH CHECK ADD  CONSTRAINT [QuestionsToSurveyQuestions] FOREIGN KEY([QuestionId])
REFERENCES [dbo].[Questions] ([QuestionId])
GO

ALTER TABLE [dbo].[SiteSurveyQuestions] CHECK CONSTRAINT [QuestionsToSurveyQuestions]
GO

ALTER TABLE [dbo].[SiteSurveyQuestions]  WITH CHECK ADD  CONSTRAINT [SurveysToSurveyQuestions] FOREIGN KEY([SurveyId])
REFERENCES [dbo].[Surveys] ([SurveyId])
GO

ALTER TABLE [dbo].[SiteSurveyQuestions] CHECK CONSTRAINT [SurveysToSurveyQuestions]
GO

ALTER TABLE [dbo].[Questions]  WITH CHECK ADD  CONSTRAINT [R409] FOREIGN KEY([ChoiceId])
REFERENCES [dbo].[Choices] ([ChoiceId])
GO

ALTER TABLE [dbo].[Choices]  WITH CHECK ADD  CONSTRAINT [R408] FOREIGN KEY([QuestionId])
REFERENCES [dbo].[Questions] ([QuestionId])
GO

ALTER TABLE [dbo].[Choices] CHECK CONSTRAINT [R408]
GO


SET ANSI_PADDING OFF
GO

GO

SET IDENTITY_INSERT [dbo].[Sites] ON 
INSERT [dbo].[Sites] ([SiteId], [Name]) VALUES (1, N'Site1')
INSERT [dbo].[Sites] ([SiteId], [Name]) VALUES (2, N'Site2')
SET IDENTITY_INSERT [dbo].[Sites] OFF

SET IDENTITY_INSERT [dbo].[Surveys] ON 
INSERT [dbo].[Surveys] ([SurveyId], [Name], [Status], [SurveyTypeId]) VALUES (1, N'SurveyAboutCats', N'Live', 0)
INSERT [dbo].[Surveys] ([SurveyId], [Name], [Status], [SurveyTypeId]) VALUES (2, N'Crime Survey', N'Live', 0)
SET IDENTITY_INSERT [dbo].[Surveys] OFF

SET IDENTITY_INSERT [dbo].[Questions] ON 
INSERT [dbo].[Questions] ([QuestionId], [InternalName], [ChoiceId], [ActiveFlag]) VALUES (1, N'Own A Cat?', NULL, 1)
INSERT [dbo].[Questions] ([QuestionId], [InternalName], [ChoiceId], [ActiveFlag]) VALUES (2, N'Own A Dog?', NULL, 1)
INSERT [dbo].[Questions] ([QuestionId], [InternalName], [ChoiceId], [ActiveFlag]) VALUES (3, N'Witnessed any crimes recently?', NULL, 1)
INSERT [dbo].[Questions] ([QuestionId], [InternalName], [ChoiceId], [ActiveFlag]) VALUES (4, N'Committed any crimes yourself recently?', NULL, 1)
SET IDENTITY_INSERT [dbo].[Questions] OFF

SET IDENTITY_INSERT [dbo].[Choices] ON 
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (1, 1, N'Yes', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (2, 1, N'No', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (3, 2, N'Yes', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (4, 2, N'No', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (5, 3, N'Yes', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (6, 3, N'Yes but I ain''t no snitch', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (7, 4, N'No', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (8, 4, N'I plead the fifth', 1)
SET IDENTITY_INSERT [dbo].[Choices] OFF

SET IDENTITY_INSERT [dbo].[Questions] ON 
INSERT [dbo].[Questions] ([QuestionId], [InternalName], [ChoiceId], [ActiveFlag]) VALUES (6, N'No cats? What is wrong with you?', 2, 1)
INSERT [dbo].[Questions] ([QuestionId], [InternalName], [ChoiceId], [ActiveFlag]) VALUES (7, N'How many cats did you feed yesterday?', 1, 1)
INSERT [dbo].[Questions] ([QuestionId], [InternalName], [ChoiceId], [ActiveFlag]) VALUES (8, N'How much do you spend on cats annually?', 1, 1)
SET IDENTITY_INSERT [dbo].[Questions] OFF

SET IDENTITY_INSERT [dbo].[Choices] ON 
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (9, 6, N'I am sorry', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (10, 7, N'1', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (11, 7, N'2-5', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (12, 7, N'6-10', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (13, 7, N'11-20', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (14, 7, N'20+', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (15, 7, N'100+', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (16, 8, N'0-100', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (17, 8, N'100-500', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (18, 8, N'500-2000', 1)
INSERT [dbo].[Choices] ([ChoiceId], [QuestionId], [InternalName], [ActiveFlag]) VALUES (19, 8, N'2000+', 1)
SET IDENTITY_INSERT [dbo].[Choices] OFF



INSERT [dbo].[SiteSurveys] ([SiteId], [SurveyId], [Name], [Status]) VALUES (1, 1, N'Site #1 Cat Survey', N'Live')
INSERT [dbo].[SiteSurveys] ([SiteId], [SurveyId], [Name], [Status]) VALUES (1, 2, N'Site #1 Crime Survey', N'Live')
INSERT [dbo].[SiteSurveys] ([SiteId], [SurveyId], [Name], [Status]) VALUES (2, 1, N'Site #2 Cat Survey', N'Live')
INSERT [dbo].[SiteSurveys] ([SiteId], [SurveyId], [Name], [Status]) VALUES (2, 2, N'Site #2 Crime Survey', N'Live')


INSERT [dbo].[SiteSurveyQuestions] ([SiteId], [SurveyId], [QuestionId], [SurveyQuestionTypeId], [ActiveFlag], [IsRequired]) VALUES (1, 1, 1, 0, 1, 0)
INSERT [dbo].[SiteSurveyQuestions] ([SiteId], [SurveyId], [QuestionId], [SurveyQuestionTypeId], [ActiveFlag], [IsRequired]) VALUES (1, 1, 2, 0, 1, 0)

GO
USE [master]
GO












-- */

Answer 1

这是一种可以避免在访问集合时从数据库中获取数据的方法：

var questions = session.Query<Question>()
    .Fetch(x => x.Choices)
    .ToList();

var choices = session.Query<Choice>()
    .Fetch(x => x.Questions)
    .ToList();

var questionsForSurvey = session.Query<SiteSurveyQuestion>()
    .Where(x => x.Site.Id == 1 && x.Survey.Id == 1)
    .ToArray();

以上显然不理想，因为它加载了所有问题和选择，但您需要通过设置 SiteSurveyQuestion 在 Question 中根据需要对其进行过滤。这样您就可以根据您的调查和网站 ID 加载您需要的问题和选择。

Answer 2

1 + N 的解决方案将建立在一个特殊的 NHibernate 优化功能之上（让我引用一下）

19.1.5. Using batch fetching

NHibernate can make efficient use of batch fetching, that is, NHibernate can load several uninitialized proxies if one proxy is accessed (or collections. Batch fetching is an optimization of the lazy select fetching strategy. There are two ways you can tune batch fetching: on the class and the collection level.

Batch fetching for classes/entities is easier to understand. Imagine you have the following situation at runtime: You have 25 Cat instances loaded in an ISession, each Cat has a reference to its Owner, a Person. The Person class is mapped with a proxy, lazy="true". If you now iterate through all cats and call cat.Owner on each, NHibernate will by default execute 25 SELECT statements, to retrieve the proxied owners. You can tune this behavior by specifying a batch-size in the mapping of Person:

<class name="Person" batch-size="10">...</class>

NHibernate will now execute only three queries, the pattern is 10, 10, 5.

You may also enable batch fetching of collections. For example, if each Person has a lazy collection of Cats, and 10 persons are currently loaded in the ISesssion, iterating through all persons will generate 10 SELECTs, one for every call to person.Cats. If you enable batch fetching for the Cats collection in the mapping of Person, NHibernate can pre-fetch collections:

<class name="Person">
    <set name="Cats" batch-size="3">
        ...
    </set>
</class>

With a batch-size of 3, NHibernate will load 3, 3, 3, 1 collections in four SELECTs. Again, the value of the attribute depends on the expected number of uninitialized collections in a particular Session.

所以，这就是 DOC。这个解决方案的伟大之处在于，我们将 在映射中进行简单查询 和优化 .

在实践中，这意味着几乎任何 one-to-many 和实体映射都应包含 BatchSize(25) （或 50 或 100... 尝试找出适合您的套件)

为了说明这一点，我调整了上面的一个映射

public QuestionMap()
{
    Table("Questions");
    // here, load this in batches by 25
    BatchSize(25);
    Id(x => x.Id, "QuestionId").GeneratedBy.Identity().UnsavedValue(0);
    Map(x => x.InternalName);
    Map(x => x.IsActive, "ActiveFlag");

    HasMany(x => x.Choices)
        .KeyColumn("QuestionId")
        .AsBag()
        .Cascade
        .AllDeleteOrphan()
        .Inverse()
        // here again
        .BatchSize(25)
        .Not.LazyLoad();
}

下一步，将取决于会话的生命周期。如果我们要使用 using(var session...){} 我们就有麻烦了。以上内容将不起作用 - 在会话之外。所有这些都必须通过会话填充。那么如何解决呢？

最好的方法是附加一些方法来遍历对象并将它们转换成一些 "DTO"

using (var session = sessionFactory.OpenSession())
{
    var questionsForSurvey = session.Query<SiteSurveyQuestion>()
        .Where(x => x.Site.Id == 1 && x.Survey.Id == 1)
        .ToArray();

    var result = new List<SiteSurveyQuestionDTO>();
    foreach(var s  in questionsForSurvey)
    {
       // here we can touch all the inner properties and collections
       // so NHibernate will load all needed data in batches
       var dto = s.doSomething();
       result.Add(dto);
    }
}

我的首选方法是实施 IClonable 并在 .Clone() 内部实现所需的内容

using (var session = sessionFactory.OpenSession())
{
    var questionsForSurvey = session.Query<SiteSurveyQuestion>()
        .Where(x => x.Site.Id == 1 && x.Survey.Id == 1)
        .ToArray()
        .Select(s => s.Clone() as SiteSurveyQuestion);
}

勾选Prototype pattern. Some more stuff about life cycle. And also, some more about batch fetching

Answer 3

使用 Radim 关于在上面设置批大小的答案，加上为站点和调查实体添加显式预加载，使我在前期的一批中获得了相当不错的 6 个查询，在第二批中获得了 2 个额外的查询访问第一个子问题。

N+1 变成了 N/300 + 1，结果是......总共 8 个查询，分为 2 个批次。

这是我更改的内容（查找带有 //new 注释的行）：

public class SiteSurveyQuestionMap : ClassMap<SiteSurveyQuestion>
{
    public SiteSurveyQuestionMap()
    {
        Table("SiteSurveyQuestions");
        CompositeId()
            .KeyReference(x => x.Site, "SiteId")
            .KeyReference(x => x.Survey, "SurveyId")
            .KeyReference(x => x.Question, "QuestionId");

        References(x => x.Site, "SiteId"). Not.LazyLoad();  //new
        References(x => x.Survey, "SurveyId").Not.LazyLoad();  //new
        References(x => x.Question, "QuestionId").Not.LazyLoad();  //new

        Map(x => x.IsActive, "ActiveFlag").Not.Nullable();
    }
}

//and so on

public class QuestionMap : ClassMap<Question>
{
    public QuestionMap()
    {
        Table("Questions");
        Id(x => x.Id, "QuestionId").GeneratedBy.Identity().UnsavedValue(0);
        Map(x => x.InternalName);
        Map(x => x.IsActive, "ActiveFlag");

        HasMany(x => x.Choices)
            .KeyColumn("QuestionId")
            .BatchSize(300)  //new
            .AsBag()
            .Cascade
            .AllDeleteOrphan()
            .Inverse()
            .Not.LazyLoad();

    }
}

public class ChoiceMap : ClassMap<Choice>
{
    public ChoiceMap()
    {
        Table("Choices");
        Id(x => x.Id, "ChoiceId").GeneratedBy.Identity().UnsavedValue(0);
        Map(x => x.InternalName);
        Map(x => x.IsActive, "ActiveFlag");
        HasMany(x => x.Questions)
            .KeyColumn("ChoiceId")
            .BatchSize(300)  //new
            .AsBag()
            .Cascade
            .AllDeleteOrphan()
            .Inverse();
    }
}

经过以上修改，NHibernate 创建的 SQL 是可以接受的。

如果有人可以改进这个答案，请改进。

在没有 N+1 的情况下通过 NHibernate 进行查询 - 包含示例

Querying via NHibernate without an N+1 - sample included

c#

nhibernate

fluent-nhibernate

19.1.5. Using batch fetching