Александр Дубина

M_News m_News = new M_News();
                m_News.Title = vm_News.Title;
                m_News.Text = vm_News.Text;
                m_News.IsActive = vm_News.IsActive;
                m_News.DateCreated = vm_News.DateCreated;

                m_News.P_CategoryNews = new List<M_CategoryNews>();

                //_context.Add(m_News);

                

                foreach (var item in vm_News.CategoriesListSelected)
                {
                    M_CategoryNews MTM = new M_CategoryNews();
                    MTM.M_News = m_News;

                    M_Category category = _context.db_Category.SingleOrDefault(z => z.IDCategory == item);
                    MTM.M_Category = category;

                    _context.Add(MTM);
                }            
                
                await _context.SaveChangesAsync();