ManualLet.cs

public void Main()
{
	var config = new MapperConfiguration(cfg =>
	{
		cfg.CreateProjection<Product, ProductArticle>()
			.ForMember(d => d.Price, o => o.MapFrom(source => source.Articles.Where(x => x.IsDefault && x.NationId == 1 && source.ECommercePublished).FirstOrDefault()));
		cfg.CreateProjection<ProductArticle, ProductModel>();
		cfg.CreateProjection<Article, PriceModel>()
			.ForMember(d => d.RegionId, o => o.MapFrom(s => s.NationId));
	});
	config.AssertConfigurationIsValid();
	var mapper = config.CreateMapper();
	var context = new ClientContext();
	context.Database.EnsureDeleted();
	context.Database.EnsureCreated();
	context.Products.Add(new UserQuery.Product {ECommercePublished=true,  Articles = new []{ new Article{IsDefault=true, NationId=1, ProductId=1}}});
	context.SaveChanges();
	var result = mapper.ProjectTo<ProductArticle>(context.Products);//.Dump();
}
public partial class ProductArticle
{
	public int Id { get; set; }
	public string Name { get; set; }
	public bool ECommercePublished { get; set; }
	public virtual ICollection<Article> Articles { get; set; }
	public Article Price;
}
public partial class Article
{
	public int Id { get; set; }
	public int ProductId { get; set; }
	public bool IsDefault { get; set; }
	public short NationId { get; set; }
	public virtual Product Product { get; set; }
}
public partial class Product
{
	public int Id { get; set; }
	public string Name { get; set; }
	public bool ECommercePublished { get; set; }
	public virtual ICollection<Article> Articles { get; set; }
}
public class PriceModel
{
	public int Id { get; set; }
	public short RegionId { get; set; }
	public bool IsDefault { get; set; }
}
public class ProductModel
{
	public int Id { get; set; }
	public PriceModel Price { get; set; }
}
class ClientContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) => optionsBuilder.UseSqlServer(
        @$"Data Source=(localdb)\mssqllocaldb;Integrated Security=True;MultipleActiveResultSets=True;Database={GetType()};Connection Timeout=300",
        o => o.EnableRetryOnFailure(maxRetryCount: 10).CommandTimeout(120));
    public DbSet<Product> Products { get; set; }
}