Gist for https://stackoverflow.com/questions/60873396/c-sharp-automapper-iqueryable-linq-2-sqlite-query-returns-only-parent-neste

Linq2SqliteMapperTest.cs

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Linq;
using System.Data.Linq.Mapping;
using System.Data.SQLite;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Runtime.Remoting.Contexts;
using System.Text;
using System.Threading.Tasks;
using AutoMapper;
using AutoMapper.QueryableExtensions;

namespace Export
{
    class Program
    {
        public static int WheelCount = 0;
        public static int CarCount = 0;
        public static string path = $@"{Directory.GetCurrentDirectory()}\car.db";
        public static string connStr = $"DataSource={path}";
        public static SQLiteConnection Conn => new SQLiteConnection(connStr);
        static void Main(string[] args)
        {
            CreateDbAndTableAndFill();

            var repo = new CarRepo();

            var allCars = repo.GetAllCarsWithoutQuery();

            var carsMissingNested = repo.Cars.ToList();

            Debugger.Break();

        }

        public class CarRepo
        {
            CarMapper mp = new CarMapper();
            CarContext context = new CarContext(Conn);

            public IEnumerable<Car> GetAllCarsWithoutQuery()
            {
                return mp.Map.Map<IEnumerable<Car>>(context.CarDtos);
            }

            public IQueryable<Car> Cars => Qry();

            private IQueryable<Car> Qry()
            {
                return mp.Map.ProjectTo<Car>(context.CarDtos);
            }
        }

        public class CarMapper
        {
            public IMapper Map => Config.CreateMapper();
            IConfigurationProvider Config;

            public CarMapper()
            {
                Config = new MapperConfiguration(cfg =>
                {
                    cfg.AddMaps(typeof(CarMapper).Assembly);
                });
                Config.AssertConfigurationIsValid();
                Config.BuildExecutionPlan(typeof(Car), typeof(CarDto));
            }
        }

        public class CarContext : DataContext
        {
            public Table<CarDto> CarDtos;

            public CarContext(IDbConnection connection) : base(connection)
            {
            }
        }

        [AutoMap(typeof(Car), ReverseMap = true)]
        [Table(Name = "Cars")]
        public class CarDto
        {
            public CarDto()
            {
            }
            [Column(IsPrimaryKey = true)]
            public Int64? Id { get; set; }
            [Column]
            public string Manufacturer { get; set; }
            [Column]
            public double Mileage { get; set; }
            [Column]
            public Int64? WheelId { get; set; }
            [Column]
            public string WheelManufacturer { get; set; }
        }

        public class Car
        {
            public Int64? Id { get; set; }
            public string Manufacturer { get; set; } = "Porsche";
            public double Mileage { get; set; }
            public Wheel Wheel { get; set; }

            public Car()
            {
                this.Id = Program.CarCount;
                Mileage = new System.Random(Program.CarCount).Next(0, 500000);

                Program.CarCount++;
            }

            public Car CreateWheels()
            {
                this.Wheel = new Wheel() { Id = Program.WheelCount };
                Program.WheelCount++;
                return this;
            }
        }

        public class Wheel
        {
            public Int64? Id { get; set; }
            public string Manufacturer { get; set; }

            public Wheel()
            {
                Id = Program.WheelCount;
                this.Manufacturer = "Continental";
            }
        }

        private static void CreateDbAndTableAndFill()
        {
            if (!File.Exists(path))
            {
                SQLiteConnection.CreateFile(path);

                var ctx = new CarContext(Conn);
                ctx.ExecuteCommand(CreateQry());

                var cars = new List<Car>();
                foreach (var n in Enumerable.Range(0, 10))
                    cars.Add(new Car().CreateWheels());

                var mp = new CarMapper();
                ctx.CarDtos.InsertAllOnSubmit(mp.Map.Map<IEnumerable<CarDto>>(cars));
                ctx.SubmitChanges();
            }
        }

        public static string CreateQry()
        {
            return
            @"CREATE TABLE IF NOT EXISTS Cars(  Id INTEGER PRIMARY KEY NOT NULL,
                                                Manufacturer TEXT,
                                                Mileage REAL,
                                                WheelId INTEGER,
                                                WheelManufacturer TEXT)";
        }

    }
}