递归Linq查询

Question

Currently I'm working with WebApi and Entity Framework, So I have 3 entities: Products, Categories and ProductCategory; their relationships are:

My problem is that Category entity has a Category Parent property, so it's recursive, my Category Controller looks like this:

public async Task<IHttpActionResult> GetCategory()
        {
            var category = await db.Category.Select(x=>new {
                x.categoryDesc,
                x.CategoryId,
                x.categoryImage,
                x.categoryName,
                x.categoryParent
            }).ToListAsync();
            return Ok(category);
        }

I'm returning an anonymous object, the propierty categoryParent its the same object as category so its recursive; when I fill the database with mock data in the Category table and call the get method, everything runs OK because I dont have any data en ProductCategory, but when I fill it(the ProductCategory table) the program crashes. MY entity classes are:

public class Category {
    public int CategoryId { set; get; }
    public string categoryName { set; get; }
    public string categoryDesc { set; get; }
    public string categoryImage { set; get; }
    public int? categoryParentId { set; get; }
    public virtual ICollection<ProductCategory> ProductCategories { set; get; }
    public virtual Category categoryParent { set; get; }
}

public class Product{
    public int ProductId { set; get; }
    public string productName { set; get; }
    public string productDesc { set; get; }
    public double productPrice { set; get; }
    public string productUrl { set; get; }
    public DateTime productPublishDate { set; get; }
    public DateTime productModifyDate { set; get; }
    public bool productStatus { set; get; }
    public int productStock { set; get; }
    public virtual ICollection<ProductCategory> ProductCategories { set; get; }    
}
public class ProductCategory : IProductCategory {
    [Required]
    [Key]
    [ForeignKey("Category")]
    [Column(Order = 1)]
    public int CategoryId { set; get; }
    [Required]
    [Key]
    [ForeignKey("Product")]
    [Column(Order = 2)]
    public int ProductId { set; get; }
    public virtual Product Product { set; get; }
    public virtual Category Category { set; get; }
}

Can you help me to fix it?, So when I return categoryParent return it recursively, Thanks

Answer 1

I'm guessing you might have better luck if you explicitly state how you want the information organized, and remove the virtual property

IQueryable<Category> category = db.Category;

var result = category.Where(w => w.categoryParentId != null)
                     .Join(category,
                           child => (int)child.categoryParentId,
                           parent => parent.CategoryId,
                           (child, parent) => new { 
                                    child.categoryDesc, 
                                    child.CategoryId, 
                                    child.categoryImage, 
                                    child.categoryName, 
                                    parent
                           }         
);

return Ok(await result.ToListAsync());

That should get you the same result as your query above, then you could remove:

public virtual Category categoryParent { set; get; }

Answer 2

Thank you very much but I found the solution: https://practiceaspnet.wordpress.com/2015/11/09/many-to-many-relationships-with-additional-fields/

I used fluent API to resolve the navigation recursive problem I had:

modelBuilder.Entity<Category>()
.HasMany(x => x.ProductCategories)
.WithRequired(x => x.Category)
.HasForeignKey(x=>x.CategoryId);

modelBuilder.Entity<Product>()
.HasMany(x => x.ProductCategories)
.WithRequired(x => x.Product)
.HasForeignKey(x => x.ProductId);

Basically the WithRequired method prevents a navigation property on the other side of the relationship so it stops the recursion