Student Table
- StudentId
- Name

Homework Table
- HomeworkId
- Name

StudentHomework Table
- StudentId
- HomeworkId

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

    public ICollection<Homework> Homeworks { get; set; }
}

class Homework
{
    public int HomeworkId { get; set; }
    public string Name { get; set; }
}

modelBuilder.Entity<Student>().HasMany(u => u.Homeworks).WithMany().Map(m =>
{
    m.MapLeftKey("StudentId");
    m.MapRightKey("HomeworkId");
    m.ToTable("StudentHomework");
});

var newStudent = new Student
{
    Name = "John Smith"
};

var homework1 = DataApi.GetData(...);
var homework2 = DataApi.GetData(...);

newStudent.Homeworks = new List<Homework>();
newStudent.Homeworks.Add(homework1);
newStudent.Homeworks.Add(homework2);

// Throws SqlException Violation of UNIQUE KEY constraint
DataApi.Save<Student>(newStudent);