C# 如何使用LINQ和实体框架6进行表连接?
我有三张表格:考试>目标>客观细节 下面是它们的样子:C# 如何使用LINQ和实体框架6进行表连接?,c#,sql,asp.net,linq,entity-framework,C#,Sql,Asp.net,Linq,Entity Framework,我有三张表格:考试>目标>客观细节 下面是它们的样子: public class Exam { public Exam() { this.Objectives = new HashSet<Objective>(); } public int ExamId { get; set; } public int SubjectId { get; set; } public virtual ICollection<Objec
public class Exam
{
public Exam()
{
this.Objectives = new HashSet<Objective>();
}
public int ExamId { get; set; }
public int SubjectId { get; set; }
public virtual ICollection<Objective> Objectives { get; set; }
}
public class Objective : AuditableTable
{
public Objective()
{
this.ObjectiveDetails = new HashSet<ObjectiveDetail>();
}
public int ObjectiveId { get; set; }
public int ExamId { get; set; }
public int Number { get; set; }
public virtual Exam Exam { get; set; }
public virtual ICollection<ObjectiveDetail> ObjectiveDetails { get; set; }
}
public partial class ObjectiveDetail
{
public int ObjectiveDetailId { get; set; }
public int ObjectiveId { get; set; }
public int Number { get; set; }
public string Text { get; set; }
public virtual Objective Objective { get; set; }
}
或者来自如下的客观细节:
var result = await db.Exams
.Where(e => e.Name == name)
var result = db.ObjectiveDetails .. ??
如何获取显示给定考试目标和目标详细信息的对象?我应该从db.examies开始还是从db.ObjectiveDetails开始?我只需要做一个连接(如果在仅关系的世界中)。但对于LINQ,我不确定从何处开始。这将返回分组的select、key和count的枚举。最后转换为list,这样就可以检索到当时的所有数据
From e in db.Exam
join o in objective on e.objectiveid = o.id
join od in objectivedetails on o.objectivedeailsId = od.id
select e
var results = simulacao.Geracao
.SelectMany(g => g.ObterCarteiras())
.SelectMany(cg => cg.CarteiraGeneticaInvestimento)
.SelectMany(cgi => cgi.HistoricoNaoPodeInvestir)
.GroupBy(hnpi => hnpi.TipoNaoPodeInvestir)
.Select(g => new { Tag = g.Key, Frequency = g.Count() })
.ToList();
这与前面的行为相同,但有两种方法可以通过LINQ获得完全相同的信息
var geracoes = (from g in simulacao.Geracao
from cg in g.ObterCarteiras()
from cgi in cg.CarteiraGeneticaInvestimento
from hnpi in cgi.HistoricoNaoPodeInvestir
group hnpi by hnpi.TipoNaoPodeInvestir into g
select new
{
TipoNaoPodeInvestir = Utilities.GetEnumDescription((EnumTipoNaoPodeInvestir)g.Key),
Count = g.Count()
}).ToList();
最后,我们可以将列表转换为json结果
return Json(geracoes, JsonRequestBehavior.AllowGet);
请注意,使用“select new”(选择新建)我们创建了一种新类型的对象,只有两个属性这是我通常在连接两个或多个表时所做的,不知道这是否是您要查找的:
var info = from p in db.Exam
join q in db.objective on p.objectiveid equals q.id
join r in db.objectivedetails on q.objectivedeailsId equals r.id
select new
{
ExamId = p.ExamId
SubjectId= p.SubjectId
ObjectiveId= q.ObjectiveId
Number = q.Number
ObjectiveDetailId = r.ObjectiveDetailId
Text = r.Text
} into x
select x;
假设实体框架已正确连接实体之间的关系,则以下内容应返回单个检查,以及相关目标和详细信息:-
var query = db.Exams.Include(e => e.Objectives.Select(o => o.ObjectiveDetails));
var examEntity = query.SingleOrDefault(e => e.ExamId == targetExamId);
上面的查询将在后台使用外部连接,因此即使没有关联的目标/详细信息,也会始终返回一个检查(如果找到)。考试作为单个对象返回,如果适用,还包括子目标
或者,下面将使用内部连接返回展平结果集:-
var query = from exam in db.Exams
from objective in exam.Objectives
from detail in objective.ObjectiveDetails
select new
{
ExamId = exam.ExamId,
SubjectId = exam.SubjectId
ObjectiveId = objective.ObjectiveId
ObjectiveNumber = objective.Number
DetailId = detail.DetailId
DetailNumber = detail.Number
Text = detail.Text
};
var examDetails = query.Where(e => e.ExamId == targetExamId).ToArray();
显式使用Linq的“join”关键字没有什么错,但是如果EF知道实体是如何关联的,则通常不需要这样做。您当前的结构似乎很好,而且如果您从db.Exam开始,那么检查是正确的方式。@KrunalPatil-但是如果我从db.Exam开始,那么我如何加入其他对象?不确定属性的正确名称,但应该是类似的,希望您能理解。两个
join
s上的条件都应该使用equals
,而不是=
。它应该是在e.objectiveid等于o.id的objective中加入o,另一个连接也是如此。耶!这就是统治的林肯!