EF6急切加载有哪些要求？

Question

我似乎设置正确，但是子对象返回null。甚至SQL输出也显示了连接（是的，设置了要加入的键）：

SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[Name] AS [Name], 
[Extent1].[Description] AS [Description], 
[Extent1].[IsVesselizable] AS [IsVesselizable], 
[Extent1].[Price] AS [Price], 
[Extent1].[CategoryId] AS [CategoryId], 
[Extent2].[Id] AS [Id1], 
[Extent2].[Name] AS [Name1]
FROM  [dbo].[MenuItems] AS [Extent1]
INNER JOIN [dbo].[MenuItemCategories] AS [Extent2] ON [Extent1].[CategoryId] = [Extent2].[Id]

这是API函数：

    // GET: api/MenuItems
    public IQueryable<MenuItem> GetMenuItems()
    {
        return db.MenuItems.Include(m => m.Category);
    }

和一些MenuItem类：

public class MenuItem
{
    public int Id { get; set; }
    [Required]
    public string Name { get; set; }

    [Required]
    public decimal Price { get; set; }

    [Required]
    public int CategoryId { get; set; }

    public MenuItemCategory Category { get; set; }
}

EF似乎应该让一切变得如此简单，但显然我在这里错过了一些东西。我不需要像这样制作最后一行，是吗？

public MenuItemCategory MenuItemCategory { get; set; }