Lesson 4 - E-shop in ASP.NET Core MVC - Relations and Repository

In the previous lesson, E-shop in ASP.NET Core MVC - Products and categories, we learned how to add additional entities to the data model, including DataAnnotation attributes. In today's ASP.NET Core tutorial, we'll add relations to these entities and update the database. Next, we'll have a look at one of the possible implementations of the Repository design pattern.

Relations

There's a M:N relation between the Product and the Category entities, because one category may contain multiple products, and one product may belong to multiple categories. We make this relationship happen by adding an association table with foreign key to both original tables. Unfortunately, EF Core (version 2.1) can't generate association tables for a Many-To-Many relationship yet, so we need to create it by ourselves.

We'll add a new CategoryProduct to the Models/ folder of the data project (we've chosen the name accordingly to the entities this table will bind together):

public class CategoryProduct
{
    public int CategoryId { get; set; }
    public virtual Category Category { get; set; }

    public int ProductId { get; set; }
    public virtual Product Product { get; set; }
}

Let's add a CategoryProduct collection to the Product and Category classes:

public virtual ICollection<CategoryProduct> CategoryProducts { get; set; }

In fact, we've now created two 1:N relationships in our classes. The first between the product and the items in the association table, the second one similarly for categories. Finally, we need to explicitly define

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