How to do a Many to one relationship / unflatten

[Newex]

How can I map an object from a database where I have an IEnumerable<DbPage> all of a single page to a Page object?

Example:

Using dapper I populate this:

public record DbPage
{
    public int ParentId { get; set; }
    public string ParentName { get; set; }
    public int? ChildId  { get; set; }
    public string? ChildName { get; set; }
}

Where each row consist of:

Row 1. ParentId = 1; ParentName = "Parent"; ChildId = 1; ChildName = "Child #1"
Row 2. ParentId = 1; ParentName = "Parent"; ChildId = 2; ChildName = "Child #2"
Row 3. ParentId = 1; ParentName = "Parent"; ChildId = 3; ChildName = "Child #3"

And I have in the business layer this Page definition:

public record Page
{
    public int ID { get; set; }
    public string Name { get; set; }
    public IEnumerable<Page> Children { get; set; }
}

I tried:

[Mapper]
public partial class MyMapper
{
    [MapProperty(nameof(DbPage.ParentID), nameof(Page.ID))]
    [MapProperty(nameof(DbPage.ParentName), nameof(Page.Name))]
    public partial Page FlatToPage(IEnumerable<DbPage> flat);
}

I also tried adding a user implemented mapping:

private Page DbParentRowToPage(IEnumerable<DbPage> parent)
{
    var group = from p in parent
                group p by p.ParentID into singleParent

                let children = singleParent.Select(x => new Page
                {
                    ID = x.ChildID.GetValueOrDefault(),
                    Name = x.ChildName ?? ""
                }).ToList()

                let first = singleParent.FirstOrDefault()
                where first is not null
                select new Page
                {
                    ID = first.ParentID,
                    Name = first.ParentName,
                    Children = children
                };

    return group.Single();
}

But I get RMG066 - No members are mapped...
I tried reading the documentation but could not get a simple console project to work. I might be missing something.

[latonz]

What does your generated code look like?

[Newex]

Here is my current mapper:

[Mapper]
public partial class MyMapper
{
    public partial Page FlatToPage(IEnumerable<DbPage> flat);

    private Page CustomMap(IEnumerable<DbPage> parent)
    {
        var group = from p in parent
                    group p by p.ParentId into singleParent

                    let children = singleParent.Select(x => new Page
                    {
                        ID = x.ChildId.GetValueOrDefault(),
                        Name = x.ChildName ?? ""
                    }).ToList()

                    let first = singleParent.FirstOrDefault()
                    where first is not null
                    select new Page
                    {
                        ID = first.ParentId,
                        Name = first.ParentName,
                        Children = children
                    };

        return group.Single();
    }
}

The generated code MyMapper.g.cs:

// <auto-generated />
#nullable enable
namespace MapFun
{
    public partial class MyMapper
    {
        [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "3.5.1.0")]
        public partial global::MapFun.Api.Page FlatToPage(global::System.Collections.Generic.IEnumerable<global::MapFun.Database.DbPage> flat)
        {
            var target = new global::MapFun.Api.Page();
            return target;
        }
    }
}

My Program.cs:

// See https://aka.ms/new-console-template for more information
using MapFun;

Console.WriteLine("Hello, World!");

var mapper = new MyMapper();
var page = mapper.FlatToPage([
    new() { ParentId = 1, ParentName = "Parent", ChildId = 1, ChildName = "Child #1" },
    new() { ParentId = 1, ParentName = "Parent", ChildId = 2, ChildName = "Child #2" },
    new() { ParentId = 1, ParentName = "Parent", ChildId = 3, ChildName = "Child #3" },
]);
Console.WriteLine(page);

Output:

Hello, World!
Page { ID = 0, Name = , Children =  }

I am probably using the library wrong. This is just a test project a put together to see if I could use it...

[latonz]

According to your sample data this looks like a group by task which is not well suited in an object to object mapper like Mapperly...