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.