使用Entity Framework嵌套循环的运行时间非常慢（使用nav属性）

Question

现在，我正在尝试为使用非常标准化的模式的调查提交程序编写一种方法。

我有一种方法旨在为一组人员生成调查，在此过程中将几个不同的EF模型链接在一起。 但是，对于最小的团队规模，此方法的运行速度极慢（对于4人团队，这需要11.2秒才能执行，而对于8人团队，则要花费103.9秒）。 经过分析，我发现以下代码块占用了75％的运行时：

 var TeamMembers = db.TeamMembers.Where(m => m.TeamID == TeamID && m.OnTeam).ToList();
                    foreach (TeamMember TeamMember in TeamMembers)
                    {
                        Employee employee = db.Employees.Find(TeamMember.EmployeeID);
                        SurveyForm form = new SurveyForm();
                        form.Submitter = employee;
                        form.State = "Not Submitted";
                        form.SurveyGroupID = surveygroup.SurveyGroupID;
                        db.SurveyForms.Add(form);
                        db.SaveChanges();

                        foreach (TeamMember peer in TeamMembers)
                        {
                            foreach (SurveySectionDetail SectionDetail in sectionDetails)
                            {
                                foreach (SurveyAttributeDetail AttributeDetail in attributeDetails.Where(a => a.SectionDetail.SurveySectionDetailID == SectionDetail.SurveySectionDetailID) )
                                {
                                    SurveyAnswer answer = new SurveyAnswer();
                                    answer.Reviewee = peer;
                                    answer.SurveyFormID = form.SurveyFormID;
                                    answer.Detail = AttributeDetail;
                                    answer.SectionDetail = SectionDetail;
                                    db.SurveyAnswers.Add(answer);
                                    db.SaveChanges();
                                }
                            }
                        }
                    }

对于如何缩减运行时间，我确实感到茫然。 这仅仅是我为拥有这么多相关实体而付出的代价吗？ 我知道联接是昂贵的操作，并且我基本上拥有3个？还是我忽略了一些效率低下的问题？

谢谢你的帮助！

编辑：根据Xiaoy312的要求，这是sectionDetails和attributeDetails的定义方式：

SurveyTemplate template = db.SurveyTemplates.Find(SurveyTemplateID);
List<SurveySectionDetail> sectionDetails = new List<SurveySectionDetail>();
List<SurveyAttributeDetail> attributeDetails = new List<SurveyAttributeDetail>();
                    foreach (SurveyTemplateSection section in template.SurveyTemplateSections)
                    {
                        SurveySectionDetail SectionDetail = new SurveySectionDetail();
                        SectionDetail.SectionName = section.SectionName;
                        SectionDetail.SectionOrder = section.SectionOrder;
                        SectionDetail.Description = section.Description;
                        SectionDetail.SurveyGroupID = surveygroup.SurveyGroupID;
                        db.SurveySectionDetails.Add(SectionDetail);
                        sectionDetails.Add(SectionDetail);
                        db.SaveChanges();

                        foreach (SurveyTemplateAttribute attribute in section.SurveyTemplateAttributes)
                        {
                            SurveyAttributeDetail AttributeDetail = new SurveyAttributeDetail();
                            AttributeDetail.AttributeName = attribute.AttributeName;
                            AttributeDetail.AttributeScale = attribute.AttributeScale;
                            AttributeDetail.AttributeType = attribute.AttributeType;
                            AttributeDetail.AttributeOrder = attribute.AttributeOrder;
                            AttributeDetail.SectionDetail = SectionDetail;
                            db.SurveyAttributeDetails.Add(AttributeDetail);
                            attributeDetails.Add(AttributeDetail);
                            db.SaveChanges();  
                        }
                    }

Answer 1

您可以改进以下几点：

1. 不要在每个Add()上保存SaveChanges() Add() ：

    foreach (TeamMember TeamMember in TeamMembers) { ... // db.SaveChanges(); foreach (TeamMember peer in TeamMembers) { foreach (SurveySectionDetail SectionDetail in sectionDetails) { foreach (SurveyAttributeDetail AttributeDetail in attributeDetails.Where(a => a.SectionDetail.SurveySectionDetailID == SectionDetail.SurveySectionDetailID) ) { ... // db.SaveChanges(); } } } db.SaveChanges(); } 

2. 考虑减少往返数据库的次数。 这可以通过以下方式完成： 它们占用大量内存
   • 使用Include()预加载您的导航属性； 要么
   • 使用ToDictionary()或ToLookup()兑现部分或整个表

3. 如果适合您的设置，请使用AddRange()甚至是EntityFramework.BulkInsert中的 BulkInsert()代替Add() ：

    db.SurveyAnswers.AddRange( TeamMembers.SelectMany(p => sectionDetails.SelectMany(s => attributeDetails.Where(a => a.SectionDetail.SurveySectionDetailID == s.SurveySectionDetailID) .Select(a => new SurveyAnswer() { Reviewee = p, SurveyFormID = form.SurveyFormID, Detail = a, SectionDetail = s, })))); 

Answer 2

使用Include可以避免SELECT N + 1问题 。

SurveyTemplate template = db.SurveyTemplates.Include("SurveyTemplateSections")
             .Include("SurveyTemplateSections.SurveyTemplateAttributes")
             .First(x=> x.SurveyTemplateID == SurveyTemplateID);

生成整个对象图，然后保存到数据库。

List<SurveySectionDetail> sectionDetails = new List<SurveySectionDetail>();
List<SurveyAttributeDetail> attributeDetails = new List<SurveyAttributeDetail>();
foreach (SurveyTemplateSection section in template.SurveyTemplateSections)
{
   SurveySectionDetail SectionDetail = new SurveySectionDetail();
   //Some code
   sectionDetails.Add(SectionDetail);

   foreach (SurveyTemplateAttribute attribute in section.SurveyTemplateAttributes)
   {
        SurveyAttributeDetail AttributeDetail = new SurveyAttributeDetail();
        //some code
        attributeDetails.Add(AttributeDetail);
   }
}
db.SurveySectionDetails.AddRange(sectionDetails);
db.SurveyAttributeDetails.AddRange(attributeDetails);
db.SaveChanges();

在循环之前加载所需的所有员工，这将避免对每个团队成员进行数据库查询。

var teamMemberIds =  db.TeamMembers.Where(m => m.TeamID == TeamID && m.OnTeam)
    .Select(x=>x.TeamMemberId).ToList(); 
var employees = db.Employees.Where(x => teamMemberIds.Contains(x.EmployeeId));

根据他们的sectionDetailId为attributeDetails创建字典，以避免在每次迭代时查询该列表。

var attributeDetailsGroupBySection = attributeDetails.GroupBy(x => x.SectionDetailId)
       .ToDictionary(x => x.Key, x => x);

将保存SurveyAnswers和SurveyForms移到循环之外：

List<SurveyForm> forms = new List<SurveyForm>();
List<SurveyAnswer> answers = new List<SurveyAnswer>();
foreach (int teamMemberId in teamMemberIds)
{
    var employee = employees.First(x => x.Id == teamMemberId);
    SurveyForm form = new SurveyForm();
    //some code
    forms.Add(form);
    foreach (int peer in teamMemberIds)
    {
         foreach (SurveySectionDetail SectionDetail in sectionDetails)
         {
              foreach (SurveyAttributeDetail AttributeDetail in 
                         attributeDetailsGroupBySection[SectionDetail.Id])
              {
                   SurveyAnswer answer = new SurveyAnswer();
                   //some code
                   answers.Add(answer);
              }
         }
    }
}
db.SurveyAnswers.AddRange(answers);
db.SurveyForms.AddRange(forms);
db.SaveChanges();

最后，如果您想更快地插入，可以使用EntityFramework.BulkInsert 。 使用此扩展名，您可以像这样保存数据：

db.BulkInsert(answers);
db.BulkInsert(forms);