Connecting multiple tables using Entity Framework (code first)

Question

I've created 2 table for my database, and connect them using foreign key, example:

internal class Student
{
    [Key]
    public int StudentId { get; set; }
    [Column(TypeName = "nvarchar(50)")]
    public string FirstName { get; set; }
    [Column(TypeName = "nvarchar(50)")]
    public string LastName { get; set; }
    [ForeignKey("GroupId")]
    public int GroupId { get; set; }
    public virtual  Group Group { get; set; }
}
internal class Group
{
    [Key]
    public int GroupId { get; set; }
    [Column(TypeName = "nvarchar(50)")]
    public string Name { get; set; }
    public virtual ICollection<Student> Students { get; set; }
}

It works just fine, but then I had to join a third table (which is connected via foreign key with Group table). So I've changed my group table a bit, and created another table called Course

internal class Group
{
    [Key]
    public int GroupId { get; set; }
    [Column(TypeName = "nvarchar(50)")]
    public string Name { get; set; }
    public virtual ICollection<Student> Students { get; set; }


    //foreign key
    [ForeignKey("Course")]
    public int CourseId { get; set; }
    public virtual Course Course { get; set; }
}

internal class Course
{
    [Key]
    public int CourseId { get; set; }
    [Column(TypeName = "nvarchar(50)")]
    public string Name { get; set; }
    [Column(TypeName = "nvarchar(250)")]
    public string Description { get; set; }

    public virtual ICollection<Group> Groups { get; set; }
}

It doesn't work as I expected, cause when I'm trying to create a migration I'm getting this error message "The property 'GroupId' cannot be configured as a navigation property. The property must be a valid entity type and the property should have a non-abstract getter and setter. For collection properties the type must implement ICollection where T is a valid entity type."

Answer 1

As the documentation indicates that, in your case, ForeignKey takes the field name of your "Group" dependent entity; https://www.entityframeworktutorial.net/code-first/foreignkey-dataannotations-attribute-in-code-first.aspx

So you must change "GroupId" to "Group" in this line;

[ForeignKey("GroupId")]