using System;

using System.Collections.Generic;

using System.Linq;

using Microsoft.EntityFrameworkCore;

public class Program

{

    public static void Main()

    {

        var options = new DbContextOptionsBuilder<ApplicationDbContext>()

            .UseInMemoryDatabase(Guid.NewGuid().ToString())

            .Options;

        using var dbContext = new ApplicationDbContext(options);

        var parent = new ParentEntity();

        parent.Children.Add(new ChildEntity());

        dbContext.Add(parent);

        dbContext.SaveChanges();

        var result = dbContext.Parents

            .Select(parent => parent.Children.Select(child => child.Id))

            .AsSingleQuery()

            .ToList();

        Console.WriteLine("Success");

    }

    public sealed class ApplicationDbContext : DbContext

    {

        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options)

        {

        }

        public DbSet<ParentEntity> Parents {get;set;}

        public DbSet<ChildEntity> Children {get;set;}

    }

    public sealed class ParentEntity

    {

        public int Id {get;set;}

        public ICollection<ChildEntity> Children {get;set;} = new HashSet<ChildEntity>();

    }

    public sealed class ChildEntity

    {

        public int Id {get;set;}

        public int ParentEntityId {get;set;}

        public ParentEntity ParentEntity {get;set;}

    }

}