Would an extension method be a reasonable way to lazy-load a class with Dapper?

I have these related classes, which are also tables in SQL: Board and BoardItemsNeeded I am wondering if using an Extension method is a reasonable way of optionally loading the child objects:

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using Dapper;
using Dapper.Contrib.Extensions;

[Table("board")]
public partial class Board
{
    [Key]
    public int id { get; set; }
    public int CustomerCuid { get; set; }
    public string ProjectName { get; set; }
    public DateTime CreateDate { get; set; }
    public short SalesEmployeeId { get; set; }
    public DateTime? LastModifiedDate { get; set; }

    public IEnumerable<BoardItemsNeeded> BoardItemsNeeded { get; set; }

    public Board GetById(int id)
    {
        using (IDbConnection cn = new SqlConnection(g.Global.CONX))
        {
            cn.Open();
            return cn.Get<Board>(id);
        }
    }
}

public static class ExtensionMethods
{
    public static Board Deep(this Board value)
    {
        using (IDbConnection cn = new SqlConnection(g.Global.CONX))
        {
            cn.Open();
            value.BoardItemsNeeded = cn.Query<BoardItemsNeeded>("SELECT * FROM board_items_needed WHERE BoardId = @id ", new { @id = value.id });
        }
        return value;
    }
}

AND

[Table("board_items_needed")]
public partial class BoardItemsNeeded
{
    [Key]
    public int id { get; set; }
    public int BoardId { get; set; }
    public short ItemType { get; set; }
    public decimal Size { get; set; }
    public string Color { get; set; }
}

I can access the full object like this:

var board = new Board();
board = board.GetById(2).Deep();

I was messing around with interfaces, generics and repositories and came back to just doing things like this. I'm trying to develop a small pattern for various business classes in the db I'm working with. In lieu of the extension method, I suppose I could make another method under the Board class to do it. I'm just not sure if I'm approaching things right. I want to get it right before committing to a pattern.