EF & Automapper. Update nested collections

Question

I trying to update nested collection (Cities) of Country entity.

Just simple enitities and dto's:

// 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; } } 

And code itself (tested in console app for the sake of simplicity):

        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();         } 

This code throws exception:

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.

even though entity after last mapping seems just fine and reflects all changes properly.

I've spent much time finding solution but got no result. Please help.

Answer 1

The problem is the country you are retrieving from database already has some cities. When you use AutoMapper like this:

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

AutoMapper is doing something like creating an IColletion<City> correctly (with one city in your example), and assigning this brand new collection to your country.Cities property.

The problem is EntityFramework doesn't know what to do with the old collection of cities.

• Should it remove your old cities and assume only the new collection?
• Should it just merge the two lists and keep both in database?

In fact, EF cannot decide for you. If you want to keep using AutoMapper, you can customize your mapping like this:

// AutoMapper Profile public class MyProfile : Profile {      protected override void Configure()     {          Mapper.CreateMap<CountryData, Country>()             .ForMember(d => d.Cities, opt => opt.Ignore())             .AfterMap(AddOrUpdateCities);     }      private void AddOrUpdateCities(CountryData dto, Country country)     {         foreach (var cityDTO in dto.Cities)         {             if (cityDTO.Id == 0)             {                 country.Cities.Add(Mapper.Map<City>(cityDTO));             }             else             {                 Mapper.Map(cityDTO, country.Cities.SingleOrDefault(c => c.Id == cityDTO.Id));             }         }     } } 

The Ignore() configuration used for Cities makes AutoMapper just keep the original proxy reference built by EntityFramework.

Then we just use AfterMap() to invoke an action doing exactly what you thought:

• For new cities, we map from DTO to Entity (AutoMapper creates a new instance) and add it to country's collection.
• For existing cities, we use an overload of Map where we pass the existing entity as the second parameter, and the city proxy as first parameter, so AutoMapper just updates the existing entity's properties.

Then you can keep your original code:

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();     }