级联下拉列表在MVC5中按需加载时出现500错误

Question

我目前拥有一个使用Entity Framework 6（ASP.NET MVC 5）从现有数据库生成的数据模型。我目前正在使用两个数据表：合同和员工。它们都与Company表相关，因此每个表都将CompanyID作为外键。当我搭建“创建合同”视图时，它当前具有EmployeeID和CompanyID的下拉列表。我要做的是让用户选择CompanyID并仅显示与该公司关联的员工。

我尝试过使用jQuery的getJSON方法，但由于循环引用而返回500错误，因为我正在返回一个JSON序列化的Employee对象，该对象具有对Company的引用（导致循环引用错误）。

员工模型：

public partial class Employee
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Employee()
    {
        this.Contracts = new HashSet<Contract>();
        this.HoleLoggings = new HashSet<HoleLogging>();
    }

    public int EmployeeID { get; set; }
    public int CompanyID { get; set; }
    public string Name { get; set; }
    public string Department { get; set; }

    public virtual Company Company { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Contract> Contracts { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<HoleLogging> HoleLoggings { get; set; }
}

合同模型：

 public partial class Contract
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Contract()
    {
        this.ContractDetails = new HashSet<ContractDetail>();
        this.Platforms = new HashSet<Platform>();
        this.ShiftReports = new HashSet<ShiftReport>();
    }

    public int ContractID { get; set; }
    public int EmployeeID { get; set; }
    public int CompanyID { get; set; }
    public Nullable<System.DateTime> StartDate { get; set; }
    public Nullable<System.DateTime> EndDate { get; set; }
    public Nullable<bool> IsApproved { get; set; }

    public virtual Company Company { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<ContractDetail> ContractDetails { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Platform> Platforms { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<ShiftReport> ShiftReports { get; set; }
    public virtual Employee Employee { get; set; }
}

这是我的合约控制器中的ActionMethod：

        public JsonResult GetEmployees(int id)
    {

        List<Employee> employees = new List<Employee>();
        var employeeList = from e in db.Employees 
                           where (e.CompanyID == id) 
                           select e.
        employees.AddRange(employeeList);
        return Json(employees, JsonRequestBehavior.AllowGet);

    }

视图的形式：

@using (Html.BeginForm()) 
{
@Html.AntiForgeryToken()

<div class="form-horizontal">
    <h4>Contract</h4>
    <hr />
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })


    <div class="form-group">
        @Html.LabelFor(model => model.CompanyID, "CompanyID",     htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownList("CompanyID", null, htmlAttributes: new { @class = "form-control" })
            @Html.ValidationMessageFor(model => model.CompanyID, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.EmployeeID, "EmployeeID", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownList("EmployeeID", new SelectList(string.Empty, "Value", "Text"), "Please select an Employee", htmlAttributes: new { @class = "form-control" })
            @Html.ValidationMessageFor(model => model.EmployeeID, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.StartDate, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.StartDate, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.StartDate, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.EndDate, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.EndDate, new { htmlAttributes = new { @class = "form-control" } })
            @Html.ValidationMessageFor(model => model.EndDate, "", new { @class = "text-danger" })
        </div>
    </div>

和jQuery：

$(function () {
    $("#CompanyID").change(function () {
        $("#EmployeeID").empty();
        var token = $('[name=__RequestVerificationToken]').val();
        $.getJSON('@Url.Action("GetEmployees")', { id: $(this).val() }, function (employees) {
            var employeesSelect = $("#EmployeeID");
            $.each(employees, function (i, employee) {
                employeesSelect.append($('<option/>', {
                    value: employee.value,
                    text: employee.text
                }));
            });
        });
    });
});

任何建议都将不胜感激！