Entity Framework 4.1 InverseProperty Attribute and ForeignKey

It is theoretically correct but SQL server (not Entity framework) doesn’t like it because your model allows single employee to be a member of both First and Second team. If the Team is deleted this will cause multiple delete paths to the same Employee entity.

This cannot be used together with cascade deletes which are used by default in EF code first if you define foreign key as mandatory (not nullable).

If you want to avoid the exception you must use fluent mapping:

public Context : DbContext
{
    public DbSet<Employee> Employees { get; set; }
    public DbSet<Team> Teams { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<Employee>()
                    .HasRequired(e => e.SecondTeam)
                    .WithMany(t => t.SecondEmployees)
                    .HasForeignKey(e => e.FirstTeamId)
                    .WillCascadeOnDelete(false);

        ...
    }
}

This will result in scenario where you must delete members of SecondTeam manually before you delete the team.