Entity Framework: linking entity A to an existing entity B - entity-framework

Question

I am rather new to the Entity Framework, so I am probably overlooking something simple here.
In my controller class I am adding a new Category entity to the database, next I am using that entity as a property on a Course entity. When I save the Course entity, the Category is saved to the database AGAIN, while I was hoping the new Course would reference the Category that was already inserted.
The (simplified) controller code that saves the first Category:
// Create and save the category
Category category = new Category {Name = "Test category"};
category = context.Categories.Add(category);
context.SaveChanges(); // The category object now has a CategoryId (the pk of the record)
// Create and save the course
Course course = new Course {
FullDescription = "This is a new course",
Name = "My new course",
Category = category // Hoping this will have EF make a link to the just inserted category
};
context.Courses.Add(course);
context.SaveChanges(); // Saves the Course AND creates a **new** Category in the db
The problem seems to be that I call saveChanges() twice. What works is removing the first call to context.saveChanges(), BUT, this is not my actual code. In my application I use a repository pattern and adding a category is done by calling categoryRepository.AddCategory(Category category). And saving the Course is done in exactly the same way, by calling courseRepo.AddCourse(Course course) that also contains a call to saveChanges().
public Category AddCategory(Category category)
{
category = context.Categories.Add(category);
context.SaveChanges();
return category;
}
I don't want to remove the calls to saveChanges() in AddCourse() and AddCategory(), because I want these to be atomic operations.
I was hoping that returning the category and subsequently using the category as a property on a new Course would link that course to the category, but apparantly that is not the case. How do I link my Course to a category that is already present in the database?

Answer 1

I'm not sure how your data model is structured but you could do something like this.
course.CategoryId = category.CategoryId;
That way you map the actual foreign key in the relationship and it does the same thing.

Answer 2

Category category = new Category {Name = "Test category"};
Course course = new Course {
FullDescription = "This is a new course",
Name = "My new course",
Category = category
};
courseRepo.AddCourse(course);
You can use only AddCourse for adding both entities, if yours repositories has the same context. If each repository has their own context, you should attach category to the courseRepo context or load entity into it (but I suppose it not suitable for you because you have different repositories).