For testing, you provide a repository object that works with data stored in a way that you can easily manipulate, such as class variables defined as collections.
The following illustration shows the difference between a business-logic class that includes data-access logic without a repository and one that uses a repository.
In one of those tutorials, you made changes in the designer manually to synchronize the data model with the database after a database change.
You need to create the relationship and update the data model so that the Entity Framework can automatically handle this relationship.
In the database diagram, right-click the table, and select Relationships.
This makes it possible for you to provide the business-logic class with a reference to any object that implements the repository interface.
For normal operation, you provide a repository object that works with the Entity Framework.