Entity Framework Core:在 WebAPI 中包含多对多相关对象 [英] Entity Framework Core: Include many-to-many related objects in WebAPI
问题描述
我不太熟悉 .NET 框架，但决定尝试 ASP.NET Core 和 EF Core.我想制作一个非常简单的 Web API 后端，但在处理多对多关系时遇到问题.
I'm not too familiar with the .NET framework but decided to try out ASP.NET Core and EF Core. I want to make a pretty simple Web API backend but I'm having trouble working with many-to-many relationships.
我知道我需要为两个实体制作一个关系表，如这篇文章中的示例所示:如何与最新的 EF Core 每晚构建建立多对多关系?
I understand that I need to make a relationship table for the two entities, as in the example from this post: How to create a many to many relationship with latest nightly builds of EF Core?
我也有我的模型构建器创建这里描述的关系 http://ef.readthedocs.io/en/latest/modeling/relationships.html#many-to-many
I also have my model builder creating the relationship as described here http://ef.readthedocs.io/en/latest/modeling/relationships.html#many-to-many
我的代码如下:
在 DBContext 中:
in 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; }
}
型号:
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都有效.我可以创建新的 MovieActor 对象并将它们添加到数据库中.一件看起来很奇怪的事情是 Movie 和 Actor 模型现在有一个 MovieActors 属性，但它始终为空，尽管连接表有条目.
This seems to be working for the most part. I can create new MovieActor objects and add them to the database. One thing that seems odd is the Movie and Actor models now have a MovieActors property but it's always null, despite the join table having entries.
我想不通的是如何在我的 WebAPI 控制器中检索这些相关对象.我的 Linq 技能很弱，所以也许我只需要复习一下，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，我希望结果在对象的数组中包含任何相关的 Actor.另一个方向也是如此，当我得到一个演员时，我想得到他们电影的数组.
The thing I can't figure out is how to retrieve these related objects in my WebAPI controllers. My Linq skills are very weak, so maybe I just need to brush up on that, but I don't know where to start. When I get a movie by ID, or get a list of all movies, I'd like the result to have any related Actors in an array on the object. The same goes in the other direction, when getting an Actor I want to get an array of their movies.
这是我试图利用这种关系的控制器方法之一:
Here is one of my controller methods trying to make use of this relationship:
// 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);
}
推荐答案
你必须按如下所示进行更改.
You have to change it as shown below.
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; }
}
}
上下文:
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; }
}
您可以导航，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();
这篇关于Entity Framework Core:在 WebAPI 中包含多对多相关对象的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
