The entity type 'List<string>' requires a primary key to be defined

Question

Getting "The entity type List<string'> requires a primary key to be defined." using .NET 6 to build a Web API.

The following is my Model class defining "Sales":

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

namespace SalesAPI.Data
{
    public class SalesItem
    {
     
        [Key]
        [Required]
        public Guid UserID { get; set; }

        [Required]
        [MinLength(5)]
        [MaxLength(75)]
        public String Title { get; set; }  = String.Empty;

        [Required]
        [MinLength(5)]
        public String Description { get; set; } = String.Empty;

        public List<String> Images { get; set; } = new List<String>();

        [Required]
        public DateTime ListingTime { get; set; }

        public String Location { get; set; } = String.Empty;

        public String ContactInfo { get; set; } = String.Empty;
    }
}

The following is my DBContext class:

using Microsoft.EntityFrameworkCore;
using SalesAPI.Controllers;
    
namespace SalesAPI.Data
{
    public class DataContext : DbContext
    {
        public DataContext(DbContextOptions<DataContext> options) : base(options) { }

        public DbSet<SalesItem> SalesItems { get; set; }
        
    }
}

Answer 1

You'll have to make sure you have a table for the

public List<String> Images { get; set; } = new List<String>

since the database isn't able to reference an unknown list size in the table created.

Change

public List<String> Images { get; set; } = new List<String>

To

public List<ImageUri> Images { get; set; } = new List<ImageUri>

and create a class.

public class ImageUri
{
    [Key]
    public int Id { get; set; }
    public string Uri { get; set; } = null!;
}

Answer 2

You can convert the List into a delimited string and use it that way instead of having to create a special class to basically hold a string.

modelBuilder.Entity<Berk>(x =>
{
    x.HasKey(y => y.BerkId);
    x.Property(y => y.TheList)
        .HasConversion(
            from => string.Join(";", from),
            to => string.IsNullOrEmpty(to) ? new List<string>() : to.Split(';', StringSplitOptions.RemoveEmptyEntries).ToList(),
            new ValueComparer<List<string>>(
                (c1, c2) => c1.SequenceEqual(c2),
                c => c.Aggregate(0, (a, v) => HashCode.Combine(a, v.GetHashCode())),
                c => c.ToList()
        )
    );
});

public class Berk
{
    [Key]
    public int BerkId { get; set; }
    public List<string> TheList { get; set; }
}

The HasConversion has the value stored as a delimited string in the db, splits it to the List when you are using it and then joins it back again when saving. Just pick a delimiter that will not be in any of your strings.

This was bugging me this afternoon, hope it helps someone else.

Answer 3

We encountered this issue when extending an EF generated model via partial class. The partial class included two properties that were not reflective of the underlying data.table and generated the error: "entity type 'list' requires a primary key to be defined". In the end, our solution was to include the [NotMapped] attribute as follows:

 [NotMapped] public List<string> Roles { get; set; } = new List<string>(); [NotMapped] public List<string[]> Claims { get; set; } = new List<string[]>();