The property 'PropertyName' could not be mapped, because it is of type 'List'

Question

I got this problem when I try to create the database with EntityFramework Core:

The property \'Rating.RatingScores\' could not be mapped, because it i

Answer 1

If the Rating class has multiple RatingScores you have a one-to-many relationship and the RatingScores property needs its own table, you therefore need to create a new class.

Class RatingScore 
{
  public int Id { get; set; }
  public decimal RtSc { get; set; }
}

Then the Rating property will look like this:

public List<RatingScore> MyRatingScores { get; set; }

However if each Rating has one RatingScore, your property should not be a collection.

public RatingScore MyRatingScore { get; Set; }

Answer 2

When you really need to put multiple values in single column can use below way

Let's say you want to create only one table for below class

public class SomeClass
{
    public Guid ID { get; set; }
    public IEnumerable<int> Values { get; set; }
}

First create a converter, which will control .net values to db values and vice versa

    using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
 
    public class IntListToStringValueConverter : ValueConverter<IEnumerable<int>, string>
    {
        public IntListToStringValueConverter() : base(le => ListToString(le), (s => StringToList(s)))
        {

        }
        public static string ListToString(IEnumerable<int> value)
        {
            if (value.IsEmptyCollection())
            {
                return null;
            }
 
            return value.Join(',');
        }

        public static IEnumerable<int> StringToList(string value)
        {  
            if (value.IsNullOrEmpty())
            {
                return null;
            }

            return value.Split(',').Select(i => Convert.ToInt32(i)); ; 
            
        }
    }

And DbContext should have below method

 protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
         .....

        var IntValueConverter = new IntListToStringValueConverter();

        modelBuilder
            .Entity<SomeClass>()
            .Property(e => e.Values)//Property
            .HasConversion(IntValueConverter);

    }

Done!! IT should work