实体框架：选择不同的对象

Question

I am using Entity Framework for my data access and manipulation. I have the following class:

public class ProductCategory
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [Required]
    [MinLength(5)]
    [MaxLength(45)]
    public string Name { get; set; }

    [Column("ParentId", TypeName = "int")]
    public int? ProductCategoryId { get; set; }
    public virtual ProductCategory Parent { get; set; }
    public ICollection<ProductCategory> SubCategories { get; set; }
}

Now I am querying using LINQ to get a list of all categories, including the related child categories (using the ParentId column) like this:

public async Task<IQueryable<ProductCategory>> GetAll()
{
    return await Task.Run(() => {
        return _db.ProductCategories.AsQueryable();
    });
}

When I run this code, I get the following result:

[
  {
    "Id": 1,
    "Name": "Computers & Software",
    "Parent": null,
    "SubCategories": [
      {
        "Id": 2,
        "Name": "Tablets & E-Readers",
        "SubCategories": [
          {
            "Id": 8,
            "Name": "Apple iPad",
            "SubCategories": []
          },
          {
            "Id": 9,
            "Name": "Samsung",
            "SubCategories": []
          },
          {
            "Id": 10,
            "Name": "Other makes",
            "SubCategories": []
          }
        ]
      }
    ]
  },
  {
    "Id": 2,
    "Name": "Tablets & E-Readers",
    "Parent": {
      "Id": 1,
      "Name": "Computers & Software",
      "Parent": null
    },
    "SubCategories": [...]
  }
]

Now, the thing is that the second object with the name "Tablets & E-Readers" already exists in the array of "SubCategories" within the first object of the array. I would like to not see this second object (DISTINCT), but cannot get this working. Does anyone know how to tackle this issue?

Answer 1

I think that there is something wrong with your data. The subcategory ( Tablets & E-Readers ) should not have a null Parent as you have in your data now. Subcategory mean that there a parent . If want only the parent category, you can try something like this:

_db.ProductCategories.Where(cat=>cat.Parent == null).AsQueryable();

May be you will get what you want. It'will not select subcategories in the main list.

Answer 2

Those extra categories are there, because they are linked in the database.

You will need to write a query to filter them out, for example using the LINQ function Except

IQueryable<ProductCatgory> parents = 
(    
    from ProductCategory pc in _db.ProductCategories
    where pc.Parent == null
    select pc
);

return (from ProductCategory pc in parents
        select new 
        { 
            Category = pc,
            SubCategories = pc.SubCategories.Except(parents)
        });