PrimitiveCollection of Dictionary

MigrationError.cs

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Design;
using Microsoft.EntityFrameworkCore.Storage.ValueConversion;

public class TestContextFactory : IDesignTimeDbContextFactory<TestContext>
{
    public TestContext CreateDbContext(string[] args)
    {
        var optionsBuilder = new DbContextOptionsBuilder<TestContext>();
        optionsBuilder.UseNpgsql("Host=localhost;Database=postgres;Username=postgres;Password=password");
        return new TestContext(optionsBuilder.Options);
    }
}

public class TestContext : DbContext
{
    public TestContext(DbContextOptions options) : base(options)
    {
    }

    public DbSet<TestEntity> Entities { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder builder)
    {
        builder.UseNpgsql("ConnectionString");
    }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<TestEntity>(b =>
        {
            b.PrimitiveCollection(e => e.AllowedTransitions)
                .ElementType()
                .HasConversion(new KeyValueListStringsConverter());
        });
    }
}

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

    public Dictionary<string, List<string>> AllowedTransitions { get; set; } = new();
}

public class KeyValueListStringsConverter : ValueConverter<KeyValuePair<string, List<string>?>?, string?>
{
    public KeyValueListStringsConverter()
        : base(value => ConvertTo(value), value => FromProvider(value))
    {
    }

    private static string? ConvertTo(KeyValuePair<string, List<string>?>? kv)
    {
        if (kv?.Key == null) return null;

        var items = kv.Value.Value == null ? null : string.Join("|", kv.Value.Value);
        return $"{kv.Value.Key}:{items}";
    }

    private static KeyValuePair<string, List<string>?>? FromProvider(string? value)
    {
        if (string.IsNullOrEmpty(value)) return null;

        var parts = value.Split(':', 2);
        if (parts.Length != 2) return new KeyValuePair<string, List<string>?>(parts[0], new List<string>());

        return new KeyValuePair<string, List<string>?>(parts[0], parts[1].Split('|').ToList());
    }
}