如何在 LINQ C# 中有效地选择导航属性

Question

我有三张桌子

表 #1：BossEmp

SNo     JobID     BossID      EMPID       StartDt
_____________________________________________________
  1        1           6          1       05-20-2016
  2        1           6          2       05-20-2016
  3        2           7          3       06-20-2016
  4        2           7          4       06-20-2016
  5        2           7          5       06-20-2016

表 #2：员工

EmpID    EmpName      Gender    DOB           Dep
_________________________________________________________    
    1    Sakthivel     M        12-11-1986    Development
    2    Regina        F        04-03-1989    Development
    3    Samantha      F        12-12-1987    Development
    4    Keerthi       F        08-18-1988    Development
    5    Pranitha      F        11-10-1985    Development
    6    Vijay         M        02-21-1987    Development
    7    Bhavana       F        12-06-1985    Development

表#3：工作

JobID     Title            Description
__________________________________________
    1     RSI              Description RSI
    2     MSI              Description MSI

在表 #1 中，BossEMP -> JobID 是 Job Table 中的 外键，BossEmp -> BossID, EmpID 是 外键 strong>员工表。

EDMX 类图是

BossEmps -> EmpID === Employee -> EmpID
BossEmps1 -> BossID === Employee -> EmpID

BossEmps -> JobID === Job -> JobID

现在我需要创建一个模型的对象

Class WorkInfo
{
    public List<Employee> EmpList { get; set; }
    public Job JobInfo { get; set; }
}

现在我需要创建List<WorkInfo>，它应该只包含女性。

请帮助我如何在 LINQ C# 中有效地选择导航属性来构造List<WorkInfo>

数据库包含超过 100 万条记录。

我尝试了以下代码：

using (var db = new EmployeeEntities()) {
                db.BossEmps.Where(b => b.Employee.Gender == "F").Select(e => new {
                    Emp = new {
                        Name = e.Employee.EmpName,
                        Id = e.Employee.EmpId
                    },
                    JobInfo = new {
                        Name = e.Job.Title,
                        Id = e.Job.JobID
                    }
                }).GroupBy(x => x.JobInfo).ToList();
            }

Answer 1

如果我猜对了，你需要所有从事相同工作的女性员工。 在这种情况下，我认为您正在寻找这样的东西：

var workInfo = context.BossEmp.Select(b => new
        {
            EmpList = b.Employes.Where(e => b.EmployeId == e.EmployeId && e.Gendar.Equals("F")),
            Job = b.Jobs.FirstOrDefault(j => b.JobId == j.JobId)
        });

这将创建具有 EmpList 和 Job 属性的对象集合。如果你需要它作为 List，你可以使用 ToList() 方法。

var employees = workInfo.EmpList.ToList();

和/或

var workInfos = workInfo.toList();

Answer 2

这是我根据您的课程快速为您整理的示例。

这应该从 BossEmp 集合中选择所有女性及其相应的工作信息和员工信息。

public class Program
{
    public static void Main(string[] args)
    {
        Program p = new Program();
    }

    public Program()
    {
        var bossEmpCollection = new List<BossEmp>()
        {
            new BossEmp() { SNo = 2, JobID = 1, BossID = 6, EmpID = 2, StartDt = DateTime.Now },
            new BossEmp() { SNo = 1, JobID = 1, BossID = 6, EmpID =  1, StartDt = DateTime.Now }
        };
        var employeeCollection = new List<Employee>()
        {
            new Employee() { EmpID = 1, EmpName = "Sakthivel", Gender = 'M', DOB = DateTime.Now, Dep = "Development" },
            new Employee() { EmpID = 2, EmpName = "Regina", Gender = 'F', DOB = DateTime.Now, Dep = "Development" }
        };
        var jobCollection = new List<Job>()
        {
            new Job() { JobID = 1, Title = "RSI", Description = "RSI" }
        };

        var workInfoCollection = from bEmp in bossEmpCollection
                                 join e in employeeCollection on bEmp.EmpID equals e.EmpID
                                 join j in jobCollection on bEmp.JobID equals j.JobID
                                 where e.Gender.Equals('F')
                                 select new WorkInfo() { EmpObject = e, JobInfo = j };

        foreach (var workInfo in workInfoCollection)
        {
            Console.WriteLine($"Emp Name: {workInfo.EmpObject.EmpName} Work Desc: {workInfo.JobInfo.Description}");
        }

        Console.ReadKey();
    }
}
public class BossEmp
{
    public int SNo { get; set; }
    public int JobID { get; set; }
    public int BossID { get; set; }
    public int EmpID { get; set; }
    public DateTime StartDt { get; set; }
}
public class WorkInfo
{
    public Employee EmpObject { get; set; }
    public Job JobInfo { get; set; }
}

public class Employee
{
    public int EmpID { get; set; }
    public string EmpName { get; set; }
    public char Gender { get; set; }
    public DateTime DOB { get; set; }
    public string Dep { get; set; }

}
public class Job
{
    public int JobID { get; set; }
    public string Title { get; set; }
    public string Description { get; set; }
}

使用此示例，将把 LINQ 构造成一个 SQL SELECT 语句，并且仅在您循环遍历 workInfoCollection 集合后执行，一次往返于数据库并返回。

每次您访问模型类上的导航属性时，后台的延迟执行都会与数据库建立连接，并且只针对您想要查看的特定项目运行查询，基本上，在我看来，调用数据库会降低效率。

如果您运行 SQL 分析器，您将能够自己验证这一点。