New column is added automaticlly when filling datagridview

I'm using linq to filling my datagrid view with this method:

public List<HopDongCungCap> XemHopDong()
        {
            return QL.HopDongCungCaps.ToList();
        }

and this is my result: http://ift.tt/1E5vR7s

My dbo.HopDongCungCap just has 1-4 column but i dont know why it appears the 5th column

Note that dbo.NhaCungCap has a relationship with dbo.HopDongCungCap

Thank you for watching my question!