c# – EF和Automapper.更新嵌套集合

只是简单的enitities和dto：

// EF Models
public class Country
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<City> Cities { get; set; }
}

public class City
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CountryId { get; set; }
    public int? Population { get; set; }

    public virtual Country Country { get; set; }
}

// DTo's
public class CountryData : IDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<CityData> Cities { get; set; }
}

public class CityData : IDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CountryId { get; set; }
    public int? Population { get; set; }
}

代码本身(为了简单起见,在控制台应用程序中测试)：

using (var context = new Context())
        {
            // getting entity from db,reflect it to dto
            var countryDTO = context.Countries.FirstOrDefault(x => x.Id == 1).ToDTO<CountryData>();

            // add new city to dto 
            countryDTO.Cities.Add(new CityData 
                                      { 
                                          CountryId = countryDTO.Id,Name = "new city",Population = 100000 
                                      });

            // change existing city name
            countryDTO.Cities.FirstOrDefault(x => x.Id == 4).Name = "another name";

            // retrieving original entity from db
            var country = context.Countries.FirstOrDefault(x => x.Id == 1);

            // mapping 
            AutoMapper.Mapper.Map(countryDTO,country);

            // save and expecting ef to recognize changes
            context.SaveChanges();
        }

此代码抛出异常：

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship,the related foreign-key property is set to a null value. If the foreign-key does not support null values,a new relationship must be defined,the foreign-key property must be assigned another non-null value,or the unrelated object must be deleted.

即使最后一次映射后的实体看起来很好,也能正确反映所有变化.

我花了很多时间寻找解决方案却没有结果.请帮忙.