实体框架：一对多？

Question

I'm juggling with One-To-Many relationship using Entity Framework:

I need to insert a bunch of data from C# objects into my SQL Server database, so I have been using Entity Framework with the Code First approach.

In my example, I have a product which have one category, yet categories can obviously belong to multiple products.

My Product-class looks like this.

    public class Product
    {
    public int ProductId { get; set; }
    public string Name { get; set; }
    public int CategoryId { get; set; }
    public Category Category { get; set; }
    }

I have tried multiple solutions for my Category class, both with and without the outcommented line but here we go:

    public class Category
{
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int CategoryId { get; set; }
    public string CategoryName { get; set; }
 // public ICollection<Product> Products { get; set; }
}

When inserting multiple products, with the same category, I get a Primary key constraint violation - which I understand why happens, but I would think Entity Framework would care of!

I have a long list of indexes, that I use to get an XML-file (based on that index) from an API. I then create an object based on that XML file. The method is called GetProductFromXML:

Foreach(int index in listOfIndexes){
   Product product = GetProductFromXML(index);
   productContext.Products.Add(product);
 }
Context.SaveChanges();

When ever I get a product, where the category already exists, I get an violation of primary key constraint-exception from my Category-table.

What I want is obviously that EF understands, that the second object should use the category from the first object.

What do I do? I find this such a simple action, that would be easily done with normal querying, but with Entity Framework I wrap my head around it and I'm going nuts!

Hope someone will give me the logic answer!

Answer 1

You're creating two new Category instances and explicitly giving them both the same CategoryId . (You're also not actually using either instance for your products, and you never set any properties on the second product. I'm assuming these are typos.)

Only create one instance:

Category category = new Category();
category.CategoryId = 1;
category.CategoryName = "categoryA";

Then use it for both of your Product instances:

Product product = new Product();
product.ProductId = 1;
product.Name = "ProductA";
product.Category = category;
context.Products.Add(product);

Product productB = new Product();
productB.ProductId = 2;
productB.Name = "ProductB";
productB.Category = category;
context.Products.Add(productB);

Edit: From a lengthy comment thread below (and the updated question which obscures the failing code behind a new method), the original problem still remains... You're creating new Category instances where you should be re-using existing instances.

Consider keeping them in a list outside of your method. As a logical guide, something like this:

// Note: If there are categories from previous runs of this logic
//       then you should fetch them from the backing data here
var categories = new List<Category>();

foreach (int index in listOfIndexes)
{
    var product = GetProductFromXML(index, categories);
    productContext.Products.Add(product);
}
Context.SaveChanges();

And witin your GetProductFromXML method:

int id = // ... get the Product ID from your XML
string name = // ... get the Product Name from your XML
//... etc.
string categoryName = // ... get the Category Name from your XML

var category = categories.SingleOrDefault(c => c.Name == categoryName);
if (category = null)
{
    // It doesn't exist yet, so add it
    category = new Category();
    category.Name = categoryName;
    category.CategoryId = // ... whatever logic you use to determine this
    categories.Add(category);
}

return new Product
{
    ProductId = id,
    Name = name,
    // etc.
    Category = category
};