实体框架：无法定义1：1关系

Question

I'd like to define relationship where Student can have only one favorite Course . I expect it would look like this in DB:

STUDENT
    ID    
    Name
    FavoriteCourseID
COURSE
    ID
    Name

How to achieve this with entity framework? I'd prefer to specify it just by attributes. I tried:

public class Student
{
    public int ID { get; set; }
    public string Name { get; set; }

    public Course FavoriteCourse { get; set; }
    public int? FavoriteCourseID { get; set; }
}

public class Class
{
    public int ID { get; set; }
    public string Name { get; set; }
}

which gave me this DB model:

STUDENT
    ID    
    Name
    FavoriteCourseID
COURSE
    ID
    Name
    StudentID // how to remove this?

Note, that it may happen that several students have the same favorite class and therefore this is unacceptable solution.

Another question: what type of relationship this is? (1:1 / 1:N ?)

Answer 1

To specify 1 to 1 relationship, it is assumed, that primary key for the related entity matches the primary key of first entity. Also you should specify a virtual property to related entity:

public class Student
{
    [Key]
    public int ID { get; set; }
    public string Name { get; set; }

    public Course FavoriteCourse { get; set; }
    public int? FavoriteCourseID { get; set; }
}

public class Class
{
    [Key]
    [ForeignKey("Student")]
    public int ID { get; set; }
    public string Name { get; set; }

    public virtual Student Student { get; set; }
}

And it will be one-to-zero-or-one relationship. Check this tutorial .

If you will mark FavouriteCourse property with RequiredAttribute , it seems, that it will result in strong one to one relationship.

It will result in adequate database structure:

STUDENT
    ID    
    Name
    FavoriteCourseID
COURSE
    ID
    Name

However, if many students could have one favourite course, this structure will be a problem, as you want one-to-many instead of one-to-one. And you will have a duplicate records in database, because one course can refer only to one student. You have to think about your db design.

Answer 2

You can try this:

public class Student
{
    public int ID { get; set; }
    public string Name { get; set; }

    [ForeignKey("FavoriteCourseId")]
    public Course FavoriteCourse { get; set; }
    public int? FavoriteCourseId { get; set; }
}

Answer 3

Normally, you define one of the following relations:

1. Optional:Optional
2. Required:Optional
3. Optional:Many
4. Required:Many
5. Many:Many

Having Required:Required is not a usual relation, inserting the first entry with such a relation needs special treatment.

I Suppose you want Required:Many as in "Each student has one favorite course but many students may chose the same favorite course".