插入具有一对多关系的新实体，而不在两个表中都创建记录

我正在使用MVC5和EF6 Code First创建Company可以包含许多Contact实体的新实体。但是，只有Company记录正在被写入数据库。

楷模：

public class Company
{
    public virtual int CompanyId { get; set; }

    [Required]
    public virtual string Name { get; set; }

    [Required]
    public virtual Address Address { get; set; }

    [Required]
    public virtual string Phone { get; set; }

    public virtual string Email { get; set; }

    // can have many Contacts
    public virtual IEnumerable<Contact> Contacts { get; set; }
}

public class Contact
{
    public virtual int ContactId { get; set; }

    [Required]
    public virtual string Title { get; set; }

    [Required]
    public virtual string Forename { get; set; }

    [Required]
    public virtual string Surname { get; set; }

    [Required]
    public virtual string Phone { get; set; }

    public virtual string Email { get; set; }

    // belongs to one Company
    public virtual Company Company { get; set; }
}

控制器：

    [HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> Create_POST([Bind(Include = "CallerType,Name,Address,Phone,Email,Contacts")] CompanyViewModel viewModel)
    {
        if (ModelState.IsValid)
        {
            // tried commented code to make sure it wasn't a problem with my repository and contact records aren't saved to database...

            //var context = new EfDbContext();

            //var company = new Company
            //{
            //    Name = viewModel.Name,
            //    Phone = viewModel.Phone,
            //    Address = viewModel.Address,
            //    Email = viewModel.Email
            //};

            //context.Companies.Add(company);

            //var contact = new Contact
            //{
            //    Company = company,
            //    Title = "mr",
            //    Forename= "f",
            //    Surname = "s",
            //    Phone = "132"
            //};

            //var contact2 = new Contact
            //{
            //    Company = company,
            //    Title = "mr",
            //    Forename = "for",
            //    Surname = "sur",
            //    Phone = "987"
            //};

            //var contacts = new List<Contact> {contact, contact2};

            //company.Contacts = contacts;

            //context.SaveChanges();


            var contacts = viewModel.Contacts.Select(c => new Contact
            {
                Title = c.Title,
                Forename = c.Forename,
                Surname = c.Surname,
                Phone = c.Phone,
                Email = c.Email
            });

            var company = new Company
            {
                Name = viewModel.Name,
                Phone = viewModel.Phone,
                Address = viewModel.Address,
                Email = viewModel.Email,
                Contacts = contacts
            };

            await _companyRepository.CreateAsync(company);

            var redirectUrl = new UrlHelper(Request.RequestContext).Action("Create", "Enquiry", new { id = company.CompanyId, callerType = viewModel.CallerType });
            return Json(new { Url = redirectUrl });
        }

        Response.StatusCode = 400;

        return PartialView("~/Views/Company/_Create.cshtml", viewModel);
    }

仓库：

    public async Task<TEntity> CreateAsync(TEntity entity)
    {
        if (entity == null) throw new ArgumentNullException(nameof(entity));
        DbContext.Set<TEntity>().Add(entity);
        await DbContext.SaveChangesAsync();
        return entity;
    }

我的印象是，DbContext将“知道”由于Contacts填充了它的集合，因此它将创建Contact链接到created的记录Company。还是我必须创建一个Contact表，因为这是其中包含外键的表，EF会“知道”创建第Company一个表？

还是有必要先实际使用保存Company实体SaveChanges()，然后将其分配给Contact记录并执行第二个记录SaveChanges()？
如果是这种情况，那么我需要使用Database.BeginTransaction()吗？

关于此的任何指导都将是很棒的，因为这是我第一次使用Entity Framework。