实体框架核心:在WebAPI中包含多对多相关对象

Question

我不太熟悉。net框架，但决定尝试一下ASP。NET Core和EF Core。我想做一个非常简单的Web API后端，但我在处理多对多关系时遇到了麻烦。

我知道我需要为两个实体创建一个关系表，就像这篇文章中的例子一样:如何使用最新的EF Core夜间构建创建多对多关系?

我也有我的模型生成器创建如下所述的关系http://ef.readthedocs.io/en/latest/modeling/relationships.html#many-to-many

我的代码是这样的:

DBContext

:

protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
        builder.Entity<MovieActor>().HasKey(x => new { x.MovieId, x.ActorId});
        builder.Entity<MovieActor>()
            .HasOne(m => m.Movie)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(m => m.MovieId);
        builder.Entity<MovieActor>()
            .HasOne(m => m.Actor)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(a => a.ActorId);
    }
    public DbSet<Movie> Movies { get; set; }
    public DbSet<Actor> Actors { get; set; }

}

模型:

namespace Test.Models
{
    public class Movie
    {
        public int MovieId { get; set; }
        public string Title { get; set; }
        public DateTime ReleaseDate { get; set; }
        public string Code { get; set; }
        public string Path { get; set; }
        public ICollection<MovieActor> MovieActors { get; set; }
    }
    public class Actor
    {
        public int ActorId { get; set; }
        public string Name { get; set; }
        public ICollection<MovieActor> MovieActors { get; set; }
    }
    public class MovieActor
    {
        public int MovieId { get; set; }
        public Movie Movie { get; set; }
        public int ActorId { get; set; }
        public Actor Actor { get; set; }
    }
}

这似乎在大多数情况下是有效的。我可以创建新的MovieActor对象并将它们添加到数据库中。看起来很奇怪的一件事是Movie和Actor模型现在有一个MovieActors属性，但它总是空的，尽管连接表有条目。

我不能弄清楚的事情是如何检索这些相关的对象在我的WebAPI控制器。我的Linq技能很弱，所以也许我只是需要温习一下，但我不知道从哪里开始。当我通过ID获取电影或获取所有电影的列表时，我希望结果在对象上的数组中有任何相关的actor。在另一个方向上也是一样的，当获得一个演员时，我想获得一个他们的电影数组。

这是我的一个控制器方法试图利用这种关系:

        // POST: api/Movie
    [HttpPost]
    public IActionResult PostMovie([FromBody] MovieBundle Moviebundle)
    {

        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }
        var _movie = _context.Movies.FirstOrDefault(m => m.Code == Moviebundle.Movie.Code);
        if(_movie == null)
        {
            //Doesn't exist! add a new one
            _movie = Moviebundle.Movie;
            _context.Movies.Add(_movie);
            _context.SaveChanges();
            foreach (Actor actor in Moviebundle.actors)
            {
                //Try to look up by name
                Actor _actor = _context.Actors.FirstOrDefault(a => a.Name == actor.Name);
                //If they don't exist, add them to the DB 
                if (_actor == null)
                {
                    _context.Actors.Add(actor);
                    _context.SaveChanges();
                    _actor = actor;
                }
                //This is what I'm using to create the M-t-M relationship
                var link = new MovieActor { Actor = _actor, Movie = _movie };
                _context.MovieActor.Add(link); //This line gives an error

            }

        } else
        {
            return new StatusCodeResult(StatusCodes.Status409Conflict);
        }


        try
        {
            _context.SaveChanges();
        }
        catch (DbUpdateException)
        {
            if (MovieExists(_movie.MovieID))
            {
                return new StatusCodeResult(StatusCodes.Status409Conflict);
            }
            else
            {
                throw;
            }
        }
        return CreatedAtAction("GetMovie", new { id = _movie.MovieID }, _movie);
    }

Answer 1

您必须按照如下所示进行更改。

模型:

namespace Test.Models
{
    public class Movie
    {
        public int MovieId { get; set; }
        public string Title { get; set; }
        public DateTime ReleaseDate { get; set; }
        public string Code { get; set; }
        public string Path { get; set; }
        public ICollection<MovieActor> MovieActors { get; set; }
    }
    public class Actor
    {
        public int ActorId { get; set; }
        public string Name { get; set; }
        public ICollection<MovieActor> MovieActors { get; set; }
    }
    public class MovieActor
    {
        public int MovieId { get; set; }
        public Movie Movie { get; set; }
        public int ActorId { get; set; }
        public Actor Actor { get; set; }
    }
}

背景:

protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
        builder.Entity<MovieActor>().HasKey(x => new { x.MovieId, x.ActorId});
        builder.Entity<MovieActor>()
            .HasOne(m => m.Movie)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(m => m.MovieId);
        builder.Entity<MovieActor>()
            .HasOne(m => m.Actor)
            .WithMany(ma => ma.MovieActors)
            .HasForeignKey(a => a.ActorId);
    }
    public DbSet<Movie> Movies { get; set; }
    public DbSet<Actor> Actors { get; set; }
    public DbSet<MovieActor> MovieActors { get; set; }
 }

可以导航，如下所示:

var actor = context.Actors
                   .Include(r => r.MovieActors)
                   .FirstOrDefault(a=>a.ActorId = 1)
var movies = actor.MovieActors.Select(c => c.Movie);

如何添加记录:

var actor = new Actor { ... };
context.Actors.Add(actor);
var movie = new Movie { ... };
context.Movies.Add(movie);
var movieActor = new MovieActor { Actor = actor , Movie = movie };
context.MovieActors.Add(movieActor);
context.SaveChanges();