.ThenInclude（）。ThenInclude（） - 在LINQ查询

Question

我似乎无法在孙子级别上加载InFrontPages的产品或在此查询中更深入。产品是为第一级子类别加载的，但对于所有更深层次，它只返回null：

var categories = await _context.ProductCategories
        .Include(e => e.ProductInCategory)
            .ThenInclude(e => e.Product)
                .ThenInclude(f => f.InFrontPages)
        .AsQueryable() // <-- Force full execution (loading) of the above
        .Where(e => e.ParentId == id) // <-- then apply the parent id filter
        // (id can be `null` (for root categories), or a category Id)
        .OrderBy(o => o.SortOrder)
        .ToListAsync();

模特：

public class ProductCategory
{
    public int Id { get; set; }
    public string Title { get; set; }
    public int SortOrder { get; set; }
    [ForeignKey(nameof(ParentCategory))]
    public int? ParentId { get; set; }

    public ProductCategory ParentCategory { get; set; } //nav.prop to parent
    public ICollection<ProductCategory> Children { get; set; } = new List<ProductCategory>();
    public ICollection<ProductInCategory> ProductInCategory { get; set; }
    public ICollection<FrontPageProduct> FrontPageProduct { get; set; }
}

public class ProductInCategory
{
    public int Id { get; set; }
    public int ProductId { get; set; }
    public int ProductCategoryId { get; set; }
    public int SortOrder { get; set; }

    public Product Product { get; set; }
    public ProductCategory ProductCategory { get; set; }
}

public class Product
{
    public int Id { get; set; }
    public int? ProductTypeId { get; set; }
    public string Title { get; set; }
    public string Info { get; set; }
    public string LongInfo { get; set; }
    public decimal Price { get; set; }
    public int? Weight { get; set; }

    // A product can belong to multiple category front pages
    public ICollection<FrontPageProduct> InFrontPages { get; set; }

    // A product can belong to multiple categories
    public ICollection<ProductInCategory> InCategories { get; set; }
}

public class FrontPageProduct
{
    public int Id { get; set; }
    public int ProductCategoryId { get; set; }
    public int ProductId { get; set; }
    public int SortOrder { get; set; }

    public Product Product { get; set; }
    public ProductCategory ProductCategory { get; set; }
}

我错过了什么？