Updating entities with Entity Framework 6

Question

I'm trying to update existing entities in my database. I want to add a m:n relationship.

This is how I try to do it:

• Get all artists from database
• Identify artists that are not yet in the database

• Save new artists

• Get all genres

• Insert ArtistGenre-relationship for all artists

• Update artists

   public void SyncArtists(ICollection<FullArtistWrapper> fullArtists) { using (var unitOfWork = UnitOfWorkFactory.CreateUnitOfWork()) { var dbArtists = unitOfWork.Repository<IArtistRepository>().GetArtists().ToList(); var newArtists = fullArtists.Where(s => dbArtists.All(d => d.ArtistId != s.Id)).ToList(); var artistsToInsert = newArtists.Select(artist => new Artist { ArtistId = artist.Id, Name = artist.Name }).ToList(); unitOfWork.Repository<IArtistRepository>().InsertEntities(artistsToInsert); unitOfWork.Commit(); // dbArtists.AddRange(artistsToInsert); var allArtists = unitOfWork.Repository<IArtistRepository>().GetArtists().ToList(); var allGenres = unitOfWork.Repository<IGenreRepository>().GetGenres(); foreach (var artist in allArtists) { var fullArtist = fullArtists.Single(f => f.Id == artist.ArtistId); var assignedDbGenres = allGenres.Where(g => fullArtist.Genres.Any(f => f == g.Name)); artist.Genres.AddRange(assignedDbGenres); } unitOfWork.Repository<IArtistRepository>().UpdateEntities(allArtists); unitOfWork.Commit(); } } 

My Entities look like this:

public class Artist : PersistenceEntity
{
    public Artist()
    {
        Genres = new List<Genre>();
    }

    [Key]
    public string ArtistId { get; set; }

    public string Name { get; set; }

    public virtual ICollection<Genre> Genres { get; set; }
}

public class Genre : PersistenceEntity
{
    [Key]
    public int GenreId { get; set; }

    public string Name { get; set; }

    public virtual ICollection<Artist> Artist { get; set; }
}

The problem is the following:

• I do have genre in the database (they get saved in an earlier step)
• The artists are saved correctly, all artists are in the database
• But the relationships don't get updated

Why is this the case? My UpdateEntities method looks like this:

public void UpdateEntities<TPersistentEntity>(ICollection<TPersistentEntity> persistentEntitiesToBeUpdated) where TPersistentEntity : PersistenceEntity
{
    persistentEntitiesToBeUpdated.ForEach(UpdateAndSave);
}

public void Update(PersistenceEntity entity)
{
    Context.Entry(entity).State = EntityState.Modified;
}

public void UpdateAndSave(PersistenceEntity entity)
{
    Update(entity);
    Context.SaveChanges();
}

Why aren't my relationships inserted?

Thanks in advance

Answer 1

What is not very well visible in your code is that you cache the artists in a private member in your class. This means that EF will now know about these entities. The proper implementation of the unit of work pattern is the following:

1. Load an entity from EF
2. Modify this instance
3. Save the instance

If you manage instances outside, EF will have a different instance associated, and it will not find any changes. the code you write marking the entity manually as modified is not needed, as EF does its own change tracking. Even if you put the state manually to modified, EF will just rescan the entity it has associated (not equal to the one you have actually modified) and will not find any changes.

The main question is why you cache these artists outside of EF. This is not good practice, in general you have more than one server / client in use, and the DB can be updated by anyone. These cached instances would never know about this. I would completely get rid of such a situation and reload entities always from EF. Then you ensure that you always get the latest state no matter who has updated the DB.