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

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 is of type 'List' which is not a supported primitive type or a valid entity type. Either explicitly map this property, or ignore it using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

Here is the class:

public class Rating
{
    public int Id { get; set; }

    public List<decimal> RatingScores { get; set; }

    public decimal Score
    {
        set => Score = value;
        get => Math.Round(RatingScores.Sum() / RatingScores.Count, 1);
    }
}

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

Ok. Here is my error and I just found what the problem is.

The property 'LogEntry.Timestamp' could not be mapped because it is of type 'Instant', which is not a supported primitive type or a valid entity type. Either explicitly map this property, or ignore it using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

Actually its fairly clear what it is trying to say.

I have overridden OnModelCreating but I missed a trivial thing.

Here is my dbcontext class.

public class ExtendedElsaMigrationsDbContext : SqlServerContext
{
    public ExtendedElsaMigrationsDbContext(DbContextOptions options) : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // I should not comment out this base method call.
        // base.OnModelCreating(modelBuilder);
        // modelBuilder.Entity<User>();

        modelBuilder.ConfigureExtendedElsaDbContext();
    }
} 

Note that I had commented out the base method call. And that is what is causing the problem.

The overridden method should be as follows.

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        //    //modelBuilder.Entity<User>();

        modelBuilder.ConfigureExtendedElsaDbContext();
    }

Summary: Don't forget to call the base method when you are overriding a method. Many times base method don't do much, so we ignore them. But some times they do. So always call the base methods.

Answer 3

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==null || value.Count()==0)
            {
                return null;
            }
 
            return value.Join(',');
        }

        public static IEnumerable<int> StringToList(string value)
        {  
            if (value==null || value==string.Empty)
            {
                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