Give a foreign key a custom name through OnModelCreating

Question

How do I stop EF Core creating a name like this? I have a table with students and workouts I have them setup for EF Core to make the link table as such. I'm using EF Core 5.09. I was following the book example

https://www.learnentityframeworkcore.com/configuration/one-to-many-relationship-configuration

But I think something went wrong if it created a name like that

public class StudentClasses
{
    public int StudentId { get; set; }
    public Students Students { get; set; }
    public int ClassId { get; set; }
    public Classes Classes { get; set; }
}

Then in OnModelCreating override, I have the following:

modelBuilder.Entity<StudentClasses>()
            .HasKey(bc => new { bc.StudentId, bc.ClassId });

modelBuilder.Entity<StudentClasses>()
            .HasOne(bc => bc.Students)
            .WithMany(b => b.StudentClasses)
            .HasForeignKey(bc => bc.StudentId);

 modelBuilder.Entity<StudentClasses>()
             .HasOne(bc => bc.Classes)
             .WithMany(c => c.StudentClasses)
             .HasForeignKey(bc => bc.ClassId);

But this results in a awful naming convention in the database.

How do I make it that it just shows ClassId as the name of the foreign key I fixed the spelling mistake in code, but I don't know how to stop the classes before it. It's a one Student can belong to many classes relationship.

My Students class

public class Students
{  
    [Key]
    public int StudentId { get; set; }
    public int? Type { get; set; }
    public string? FirstName { get; set; }
    public string? Surname { get; set; }
    public DateTime? DOB { get; set; }
    public decimal? Weight { get; set; }
    public decimal? Height { get; set; }
    public int? Gender { get; set; }
    public string? Age { get; set; }

    public ICollection<StudentWorkouts> StudentWorkouts { get; set; }
    public ICollection<StudentClasses> StudentClasses { get; set; }
    public bool? IsDeleted { get; set; }
    public bool? IsActive { get; set; }
    public string? CreatedBy { get; set; }
    public string? LastModifiedBy { get; set; }
    public DateTime? LastUpdatedDate { get; set; }
    public DateTime? CreatedDate { get; set; }
}

My Classes class - no pun intended:

public class Classes
{
    [Key]
    public int ClassedId { get; set; }
    public Guid? UserId { get; set; }
    public ICollection<Students> Students { get; set; }
    public DateTime? StartDate { get; set; }
    public DateTime? EndDate { get; set; }
    public ICollection<StudentClasses> StudentClasses { get; set; }
    public bool? IsDeleted { get; set; }
    public bool? IsActive { get; set; }
    public string? CreatedBy { get; set; }
    public string? LastModifiedBy { get; set; }
    public DateTime? LastUpdatedDate { get; set; }
    public DateTime? CreatedDate { get; set; }
}

Answer 1

The bit that looks to be screwing you up is:

public ICollection<Students> Students { get; set; }

The relationship that EF will be trying to establish here is a key in Students to point back to classes. Hence a ClassesClassId derived from the ClassId in the Classes entity.

If you want the students for a class, you should not be doubling up between Classes.StudentClasses and Classes.Students.

Your example appears to be a textbook many-to-many relationship.

If you are using EF Core 5 then you can now avoid needing to declare the StudentClass entity entirely:

public class Student
{
    // ...
    public virtual ICollection<Classes> Classes { get; set; } = new List<Classes>();
    // ...
}

public class Classes
{
    // ...
    public virtual ICollection<Student> Students { get; set; } = new List<Students>();
    // ...
}

EF will automatically generate the joining table, or you can map an explicit joining table/entity.

EF Core 2/3.1 required joining entities to be declared and directly referenced, much like the example you are following. This isn't a many-to-many relationship but rather a 1-to-many-to-1 relationship.

A more up to date reference if you are using EF Core 5: https://www.thereformedprogrammer.net/updating-many-to-many-relationships-in-ef-core-5-and-above/

Explicit mapping and use of a joining entity is usually only required if you want to access non-key properties on the joining entity. For example if you have properties or relationships specific to a StudentClass entity that you want to access, or need to populate when working with students and/or classes. If the table is merely a StudentId & ClassId, then it's typically preferable to just work with a student's classes collection through Student.Classes rather than Student.StudentClasses.Select(sc => sc.Class).ToList().

If you're using EF Core 2 or 3.1 then you're essentially stuck with using the joining entity 1-to-many-to-1 relationship. I would highly recommend pushing to update to EF Core 5 as IMHO EF Core 3.1 is still a dog's breakfast of unimplemented features from EF6. :)