Replacement for MemberMapper and MemberMapperAttribute in LinqToDB

[alexey-leonovich]

I'm migrating from BLToolkit to LinqToDB. Unfortunatelly that code was not written by me and I've never been used BLToolkit before.
In one class MemberMapper and MemberMapperAttribute are used like that:

[Table(Name = "Events")]
public sealed class Event:
{
    [Column(CanBeNull = false, DbType = "uniqueidentifier", IsPrimaryKey = true)]
    public new Guid Id  { get; set; }

    [Column]
    public DateTime Date { get; set; }

    [Column(DbType = "uniqueidentifier")]
    [MemberMapper(typeof(GenericMemberMapper<IUser>))]
    public IUser User{ get; set; }
}

public sealed class GenericMemberMapper<TProperty> : MemberMapper
{
    public override object GetValue(object o)
    {
        return MyMappingSchema.Instance.ConvertParameterValue(base.GetValue(o), typeof(TProperty));
    }

    public override void SetValue(object o, object value)
    {
        base.SetValue(o, (TProperty)(MyMappingSchema.Instance.ConvertChangeType(value, typeof(TProperty))));
    }

    public override void Init(MapMemberInfo mapMemberInfo)
    {
        if (typeof(IEntity).IsAssignableFrom(typeof(TProperty)))
        {
            mapMemberInfo.DbType = DbType.Guid;
            mapMemberInfo.IsDbTypeSet = true;
        }

        base.Init(mapMemberInfo);
    }
}

As I can see - there is no MemberMapper in LinqToDB. But if I comment [MemberMapper(typeof(GenericMemberMapper<IUser>))] line - I get an error when trying to select Events from database by some User:

Expression 'e.User' is not a Field.

What is the replacement for MemberMapper and MemberMapperAttribute in LinqToDB? Is there some tutorial/docs section? I didn't find any migration guide, sorry.

[igor-tkachev]

This attribute tells BLToolkit how to convert database uniqueidentifier column to your IUser object. There should be this logic somewhere in your code.

It can be replaced with MappingSchema SetConvertExpression/SetConverter methods.

[alexey-leonovich]

I couldn’t figure out in which cases I should use SetConvertExpression and in which SetConverter. I tried both but couldn't get them to work. Here is a highly simplified sample code (I deliberately avoid using relationships between tables in purpose of converters testing):

public interface IRole
{
    Guid Id { get; }
    string Name { get; }
}

public class Role : IRole
{
    [Column(IsPrimaryKey = true)]
    public Guid Id { get; set; }

    public string Name { get; set; }
}

public class User
{
    [Column(IsPrimaryKey = true)]
    public Guid Id { get; set; }

    public string Name { get; set; }

    [Column(DbType = "uniqueidentifier", CanBeNull = true)]
    public IRole Role { get; set; }
}

internal class Program
{
    static Role _adminRole = new() { Id = Guid.Parse("11111111-e20c-4288-a1ab-186b18668352"), Name = "Admin" };
    static Role _userRole = new() { Id = Guid.Parse("22222222-e20c-4288-a1ab-186b18668352"), Name = "User" };
    static List<Role> _roles = new() { _adminRole, _userRole};

    static User _admin = new() { Id = Guid.Parse("44444444-e20c-4288-a1ab-186b18668352"), Name = "Dan", Role = _adminRole };
    static User _user = new() { Id = Guid.Parse("55555555-e20c-4288-a1ab-186b18668352"), Name = "Adam", Role = _userRole };
    static User _userNoRole = new() { Id = Guid.Parse("66666666-e20c-4288-a1ab-186b18668352"), Name = "Noname", Role = null };
    static List<User> _users = new() { _admin, _user, _userNoRole };

    static void Main(string[] args)
    {
        using (var db = new DataConnection(new DataOptions().UseSqlCe(@"Data Source=D:\users.sdf")))
        {
            var sc = db.MappingSchema;
            sc.SetConverter<Guid, IRole>(id => _roles.SingleOrDefault(r => r.Id == id));
            sc.SetConverter<IRole, Guid>(role => role?.Id);
            
            if (!db.GetTable<Role>().Any())
                db.GetTable<Role>().BulkCopy(_roles);
            if (!db.GetTable<User>().Any())
                db.GetTable<User>().BulkCopy(_users);
            
            Role noRole = null;
            foreach (var u in db.GetTable<User>().Where(u => u.Role == _adminRole || u.Role == noRole))
                Console.WriteLine($"{u.Id} {u.Name} {u.Role?.Name}");
        }
        Console.ReadKey();
    }
}

But on db.GetTable<User>().BulkCopy(_users); line I get an exception:

System.ArgumentException: 'No mapping exists from DbType DataModel.Role to a known SqlCeType.'

Looks like I'm setting up the converters incorrectly. What is my mistake? Thank you.

[alexey-leonovich]

When I used sc.SetConvertExpression<IRole, DataParameter>(role => new DataParameter(null, role?.Id), false); instead of sc.SetConverter<IRole, Guid>(role => role?.Id); exception has gone. Looks like that was my mistake.