类型'；System.InvalidOperationException'；在EntityFramework

我正在尝试制作部门名称的下拉列表。我正在使用MVC5。我看到了太多关于堆栈溢出的解决方案，但我从未找到与MVC5相关的有价值的解决方案。

Database Name : AppraisalDBContext
Table Name :  Department
Column Name : deptID (Primarykey)
              deptName (department name)
              Description 

我得到这个错误：

An exception of type 'System.InvalidOperationException' occurred in  EntityFramework.dll but was not handled in user code
Additional information: The model backing the 'AppraisalDBContext' context has changed since the database was created.Consider using Code First Migrations to update the database (http://go.microsoft.com/fwlink/?

代码：

控制器类名称（DepartmentController.cs）：

public ActionResult Index(string depName)
{
    var DeptLst = new List<string>();          
    var GenreQry = from d in db.Department
                   orderby d.deptName
                   select d.deptName;
    DeptLst.AddRange(GenreQry);                // Here I am confusing
    ViewBag.depName = new SelectList(DeptLst); // Here I am confusing
    var depts = from m in db.Department        // Here I am confusing
                select m;

    return View(depts);
}

型号类别名称（Departments.cs）:

namespace appraisalProject.Models
{
    [Table("Department")]
    public class Departments
    {
        [Key]
        public int deptId { get; set; }
        public string deptName { get; set; }
        public string Description { get; set; }
     }
}

模型类名（AppraisalDBContext.cs）:

namespace appraisalProject.Models
{
    public class AppraisalDBContext:DbContext
    {
        public DbSet<Departments> Department { get; set; }
    }
}

Index.chtml:

@using (Html.BeginForm())
{
<p>
    Genre: @Html.DropDownList("depts", "All")
    <input type="submit" value="Filter" />
</p>
}