Entity Framework attach trouble in many to many update scenario

问题

I have a scenario where I wish to update a movie entity and its many to many relationship with Genres.

The navigation property Genres in movie contains stub Genre objects that only contain the GenreID because I want to save querying the DB for all genres.

See the code below, its fairly self explanatory. The problem is that I need to attach my "Stub" genres to the context so the EF will only modify the M:M relationship and not try to create new Genre records. This causes an error if a genre is already attached to the context from when I load the movie's current genres. It can't track multiple objects with the same keys.

How should this be handled? Is there a way to check if the context is already tracking the entity, or is there a better solution to this problem?

The code is below:

public override void Update(Movie movie)
        {
            //Backup new genres before clearing genres from movie
            var newGenres = movie.Genres;
            movie.Genres = new List<Genre>();


            _ctx.Entry(movie).State = System.Data.EntityState.Modified;

            //Load movie's current genres from DB and remove them
            _ctx.Entry(movie).Collection(m => m.Genres).Load();
            movie.Genres.Clear();

            //Add new genres to movie
            foreach (var genre in newGenres)
            {
                _ctx.Genres.Attach(genre); //Problem if genre already attached
                movie.Genres.Add(genre);
            }
        }

Thanks for any help.

回答1:

You could try to avoid to attach the stubs if an entity with the same key is already attached:

public override void Update(Movie movie)
{
    //Backup new genres before clearing genres from movie
    var newGenres = movie.Genres;
    movie.Genres = new List<Genre>();

    _ctx.Entry(movie).State = System.Data.EntityState.Modified;

    //Load movie's current genres from DB and remove them
    ctx.Entry(movie).Collection(m => m.Genres).Load();
    var currentGenres = movie.Genres.ToList();
    movie.Genres.Clear();

    //Add new genres to movie
    foreach (var genre in newGenres)
    {
        var currentGenre = currentGenres.SingleOrDefault(g => g.Id == genre.Id);
        if (currentGenre != null)
            movie.Genres.Add(currentGenre);
        else
        {
            _ctx.Genres.Attach(genre);
            movie.Genres.Add(genre);
        }
    }
}

来源：https://stackoverflow.com/questions/8522596/entity-framework-attach-trouble-in-many-to-many-update-scenario