public class SchoolEntities : DbContext 
{ 
    protected override void OnModelCreating( DbModelBuilder modelBuilder ) 
    { 
      modelBuilder.Entity<aspnet_UsersInRoles>().HasMany(i =>i.Users).WithRequired().WillCascadeOnDelete(false); 
    }
}

modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();

public class SchoolEntities : DbContext 
{ 
    public DbSet<Course> Courses { get; set; } 
    public DbSet<Department> Departments { get; set; } 
    public DbSet<Instructor> Instructors { get; set; } 
    public DbSet<OfficeAssignment> OfficeAssignments { get; set; } 
 
    protected override void OnModelCreating(DbModelBuilder modelBuilder) 
    { 
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>(); 
    } 
}