EntityFramework 中的遞歸查詢拋出 NotSupportedException

Question

我想在EntityFramework 6.2.0 中構建一個遞歸查詢，以獲取員工及其所有“直接”（一個級別）以及所有其他下屬的所有層次結構。

我的觀點是使用List<IQueryable<T>>來構建一個完整的查詢，然后通過一次對數據庫的往返旅行只運行一次。

這是我的嘗試：

private static List<IQueryable<Employee>> queryables = new List<IQueryable<Employee>>();

static void Main(string[] args)
{
    using (var db = new EmployeeContext())
    {
        IQueryable<Employee> managers = db.Employee.Where(x => x.Id == 1);
        GetSlaves(managers);

        // the System.NotSupportedException occurs in this line
        IQueryable<Employee> employees = queryables.Aggregate(Queryable.Union);

        // but throws here
        var res = (
            from e in employees
            join d in db.EmployeeDoc on e.Id equals d.EmployeeId
            select new { e.Id, e.EmployeeName, d.DocNumber }).ToList();
    }

    Console.ReadLine();
}

static void GetSlaves(IQueryable<Employee> managers)
{
    if (managers != null)
    {
        queryables.Add(managers);

        foreach (var m in managers)
        {
            Console.WriteLine($"{m.Id} {m.EmployeeName} {m.Position}");
            GetSlaves(m.Slaves.AsQueryable());
        }
    }
}

但是我得到一個System.NotSupportedException ：'無法創建一個類型為'EF6.Employee'的常量值。 在此上下文中僅支持原始類型或枚舉類型。

上面的 C# 代碼試圖替換以下 SQL 代碼：

declare @managerId int = 1

;with Employees(Id, EmployeeName)
as
(
    select e.Id, e.EmployeeName from Employee as e
    where Id = @managerId
    union all 
    select e.Id, e.EmployeeName from Employee as e
    inner join  Employees as em on e.ManagerId = em.Id
)

select e.Id, e.EmployeeName, d.DocNumber
from Employees e
inner join EmployeeDocuments d on e.Id = d.EmployeeId

更新：

這是我使用的 SQL 腳本：

create table Employee
(
    Id int not null identity(1,1) primary key,
    EmployeeName varchar(20),
    Position varchar(30),
    ManagerId int constraint FK_Employee foreign key references Employee(Id)
)

insert into Employee (EmployeeName, Position, ManagerId) values
('John', 'CEO', NULL),
('Marry', 'Head of sales division', 1),
('Mike', 'Head of HR division', 1),
('Jack', 'Sales manager', 2),
('Olivia', 'Sales manager', 2),
('Sophia', 'Sales manager', 2),
('Nadya', 'HR manager', 3),
('Tim', 'HR manager', 3),
('Jim', 'Salesman', 4),
('Sergey', 'Salesman', 4),
('Dmitry', 'Salesman', 5),
('Irina', 'Salesman', 5),
('William', 'Assistant', 8)

select * from Employee

Create table EmployeeDocuments
(
    Id int not null identity(1,1) primary key,
    DocNumber varchar(20),
    EmployeeId int not null constraint FK_Docs_Employee foreign key references Employee(Id)
)

insert into EmployeeDocuments (DocNumber, EmployeeId) values
('1/2019-01-15', 1), ('3/2019-02-25', 3), ('4/2019-01-31', 4), ('9/2019-02-28', 9)

select * from EmployeeDocuments

這是Employee類：

[Table("Employee")]
public partial class Employee
{
    public Employee()
    {
        Slaves = new HashSet<Employee>();
    }

    public int Id { get; set; }

    [StringLength(20)]
    public string EmployeeName { get; set; }

    [StringLength(30)]
    public string Position { get; set; }

    public int? ManagerId { get; set; }

    public virtual ICollection<Employee> Slaves { get; set; }

    public virtual Employee Manager { get; set; }

    public virtual ICollection<EmployeeDoc> EmployeeDocs { get; set; }
}

Answer 1

錯誤是由於查詢是靜態的。 當代碼非常簡單時，不確定為什么需要查詢。 見下文

    class Program
    {
        static void Main(string[] args)
        {
            Employee.GetSlaves();

        }
    }
    public partial class Employee
    {
        public static List<Employee> employees = new List<Employee>() {
            new Employee() {Id = 1, EmployeeName =  "John",  Position =  "CEO", ManagerId = null},
            new Employee() {Id = 2,EmployeeName =  "Marry", Position =  "Head of sales division", ManagerId = 1},
            new Employee() {Id = 3,EmployeeName =  "Mike", Position =  "Head of HR division", ManagerId = 1},
            new Employee() {Id = 4,EmployeeName =  "Jack", Position =  "Sales manager", ManagerId = 2},
            new Employee() {Id = 5,EmployeeName =  "Olivia", Position =  "Sales manager", ManagerId = 2},
            new Employee() {Id = 6,EmployeeName =  "Sophia", Position =  "Sales manager", ManagerId = 2},
            new Employee() {Id = 7,EmployeeName =  "Nadya", Position =  "HR manager", ManagerId = 3},
            new Employee() {Id = 8,EmployeeName =  "Tim", Position =  "HR manager", ManagerId = 3},
            new Employee() {Id = 9,EmployeeName =  "Jim", Position =  "Salesman", ManagerId = 4},
            new Employee() {Id = 10,EmployeeName =  "Sergey", Position =  "Salesman", ManagerId = 4},
            new Employee() {Id = 11,EmployeeName =  "Dmitry", Position =  "Salesman", ManagerId = 5},
            new Employee() {Id = 12,EmployeeName =  "Irina", Position =  "Salesman", ManagerId = 5},
            new Employee() {Id = 13,EmployeeName =  "William", Position =  "Assistant", ManagerId = 8}
        };

        public static void GetSlaves()
        {
            Employee ceo = employees.Where(x => x.ManagerId == null).First();
            GetSlavesRecursive(ceo);
        }

        public int Id { get; set; }


        public string EmployeeName { get; set; }


        public string Position { get; set; }

        public int? ManagerId { get; set; }

        public virtual ICollection<Employee> Slaves { get; set; }

        public virtual Employee Manager { get; set; }

        //public virtual ICollection<EmployeeDoc> EmployeeDocs { get; set; }

        static void GetSlavesRecursive(Employee manager)
        { 
            manager.Slaves  = employees.Where(x => x.ManagerId == manager.Id).ToList();
            foreach (Employee slave in manager.Slaves)
            {
                GetSlavesRecursive(slave);
            }
        }
    }