In one of the preceding examples, we retrieved the category name of a product using the following expression:

p.Category.CategoryName == "Beverages"

Even though there is no field called categoryname in the Products table, we can still get the category name of a product because there is an association between the Products and Category tables. In the Northwind.edmx design pane, click on the line that connects the Products and Categories tables and you will see all of the properties of the association. Note that its Referential Constraint property is Category.CategoryID -> Product.CategoryID, meaning that category ID is the key field to link these two tables.

Because of this association, we can retrieve the category for each product and also retrieve products for each category.