Insert a entity that contains a collection navigation property - insert

Question

I have two classes:
public class Student
{
public int StudentID { get; set; }
public List<Course> Courses { get; set; }
}
public class Course
{
public int CourseID { get; set; }
public string Name { get; set; }
}
And a context:
public class DBContext: DbContext
{
public DbSet<Student> Students { get; set; }
public DbSet<Course> Courses { get; set; }
public DBContext(DbContextOptions<DBContext> options) : base(options) { }
}
Courses and students can exist independently of each other. In other words, I can create a Course first, then create a Student and associate them with the course.
For example:
var options = new DbContextOptionsBuilder<DBContext>().UseSqlite("Data Source=.\\test.db;");
Course course = null;
using(var context = new DBContext(options.Options))
{
context.Database.EnsureCreated();
}
using (var context = new DBContext(options.Options))
{
course = context.Courses.Add(new Course { Name = "Algebra" }).Entity;
context.SaveChanges();
}
using (var context = new DBContext(options.Options))
{
var student = context.Students.Add(new Student { Courses = new List<Course> { course } });
context.SaveChanges();
}
However, in the last context scope, I get an unique constraint error. I know that some people say that I need to do everything within the same context. But this will be nearly impossible when using, for example, a WebAPI microservice architecture (StudentService and CourseService).
So how do I handle this?

Answer 1

The problem here is that in the last using block context doesn't know that course instance has been saved already in a previous operation.
I did a small test and I saw that course doesn't have the assigned Id from the database after SaveChanges is called in second using block.
In general the most common way to perform this is to fetch the entity from the same context where we need to save the data. So this will do the trick.
Course course = new Course { Name = "Algebra" };
using (var context = new DBContext(options.Options))
{
context.Database.EnsureCreated();
}
using (var context = new DBContext(options.Options))
{
context.Courses.Add(course);
context.SaveChanges();
}
using (var context = new DBContext(options.Options))
{
var courseFromDb = context.Courses.Single(c => c.CourseID == course.CourseID);
var student = context.Students.Add(new Student { Courses = new List<Course> {courseFromDb}});
context.SaveChanges();
}