The entity type 'Microsoft.AspNet.Mvc.Rendering.SelectListGroup' requires a key to be defined

Question

Not sure what happened but I am getting the following error while attempting to pull up any view in my web app. The code is auto generated by visual studio and I am not getting any errors before building. Using ASP.Net MVC 6, EF7.

An exception of type 'System.InvalidOperationException' occurred in EntityFramework.Core.dll but was not handled in user code

Additional information: The entity type 'Microsoft.AspNet.Mvc.Rendering.SelectListGroup' requires a key to be defined.

Here is the line the code is erroring out on.

public IActionResult Index()
{
    var schoolContext = _context.Schools
                                .Include(s => s.District)
                                .Include(s => s.Location)
                                .Include(s => s.Tier);
    return View(schoolContext.ToList());
}

After some searching I can't figure out exactly what I need to fix. This was working at one point. Not sure what changed.

The view does have a defenition

@model IEnumerable<School>

As requested here is the School model

public class School
{
    //Original Fields
    public int SchoolId { get; set; }

    [Display(Name = "Name")]
    public string SchoolName { get; set; }
    
    [Display(Name = "Date Added")]
    public DateTime SchoolDateAdded { get; set; }

    [Display(Name = "Last Update")]
    public DateTime SchoolLastUpdate { get; set; }
    
    [Display(Name="Updated By")]
    public string SchoolUpdatedBy { get; set; }

    //Referance Fields
    public int DistrictId { get; set; }
    public IEnumerable<SelectListItem> DistrictList { get; set; }
    public int LocationId { get; set; }
    public IEnumerable<SelectListItem> LocationList { get; set; }
    public int TierId { get; set; }
    public IEnumerable<SelectListItem> TierList { get; set; }

    //Navigation Property
    public District District { get; set; }
    public Location Location { get; set; }
    public Tier Tier { get; set; }
}

Answer 1

These IEnumerable<SelectListItem>s should not be part of your EF model. Remember the single responsibility principle. Keep any UI framework away from your DAL implementation. Use a view model representing a School.

As for the error, from EF's point of view, School has a 1-n association with SelectListItem, so it tries to make it part of its mapping schema. But each mapped type needs a primary key, which of course isn't mapped, and EF can't infer any.

A quick, but dirty, fix would be to exclude the properties from being mapped by the [NotMapped] attribute, but a better segregation of your code is the real remedy.