ASP.NET实体框架插入（如果不存在）

Question

嘿男孩和女孩:)

我目前正在尝试实施ac＃/ asp.net代码优先的EntityFramework数据库应用程序（蔚蓝），该应用程序可以保存电影列表。 每次我向应用程序询问电影时，它都会执行以下操作：

1. 查询数据库
2. 如果在数据库中找不到，则查询TMDB
3. 添加到数据库（无重复项）

我有一个有效的查询，我有一个有效的TMDB解析器，我可以向数据库中添加内容。 当我尝试在自己的关系中没有重复项时，就会出现问题。

我的设置：

电影

public partial class Movies
    {
        public Movies() { this.Genres = new HashSet<Genres>(); /* [...] */ }

        [Key]
        public int Movie_ID { get; set; }
        // [...]
        public virtual ICollection<Genres> Genres { get; set; }
    }

体裁

public partial class Genres
{
    public Genres() { }

    [Key]
    public int Genre_ID { get; set; }
    public string Genre_Name { get; set; }
}

数据库访问类别

==>这是我的问题出现的地方<==

Movies daMov = new Movies();
// [...]
foreach ( string genre in movieGenres )
        {
            Genres daGenre = await dataBaseContext.Genres.Where(
                        m =>
                            m.Genre_Name == genre
                        ).FirstOrDefaultAsync( );       // query genre from db

            // doesn't exist - firstOrDefault() returns default
            if ( daGenre == null || object.Equals( daGenre, default( Genres ) ) )               
            {
                daGenre = new Genres(); // create new in db
                daGenre.Genre_Name = genre;
                daMov.Genres.Add( daGenre ); // works fine, just that it creates a new genre
            }
            else
            {
                // here i've been googling around for some time and found the following 
                // (all with the same error)

                //db.Genres.Attach( daGenre ); // don't create new??
                //daMov.Genres.Add( daGenre ); // should work?
                //db.Entry( daGenre ).State = EntityState.Modified; // no it doesn't
            }
        }
// [...]
db.Movies.Add( daMov );
await db.SaveChangesAsync(); // DBHelper.cs:line 140

这就是我的ASP.NET WebAPI抛出的（清除）错误消息：

ExceptionMessage: 
An error occurred while saving entities that do not expose foreign key properties for their relationships. 
The EntityEntries property will return null because a single entity cannot be identified as the source of the exception. 
Handling of exceptions while saving can be made easier by exposing foreign key properties in your entity types. See the InnerException for details.,
ExceptionType: System.Data.Entity.Infrastructure.DbUpdateException,

StackTrace:    
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at dotNETproject1.WebAPI.App_Code.DBHelper.<queryIMDB>d__0.MoveNext() in App_Code\DBHelper.cs: line 140

InnerException: 
    ExceptionMessage: Store update, insert, or delete statement affected an unexpected number of rows (0). 
    Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries.,
    ExceptionType: System.Data.Entity.Core.OptimisticConcurrencyException,

Answer 1

首先，您需要添加public virtual ICollection<Movies> Movies{ get; set; } public virtual ICollection<Movies> Movies{ get; set; } public virtual ICollection<Movies> Movies{ get; set; } ，以确保多对多关系。

编辑

我也看到您同时拥有db和dataBaseContext ，这是错误的。 您必须使用相同的数据库上下文

我认为类似的方法应该起作用：

Movies daMov = new Movies();
// [...]
foreach ( string genre in movieGenres )
        {
            Genres daGenre = await db.Genres.FirstOrDefaultAsync(
                   m => m.Genre_Name == genre);

            // doesn't exist - firstOrDefault() returns default
            if ( daGenre == null || object.Equals(daGenre, default(Genres)))               
            {
                daGenre = new Genres(); // create new in db
                daGenre.Genre_Name = genre;
            }
            daMov.Genres.Add( daGenre );
            db.Movies.Add(daMov);
            await db.SaveChangesAsync();

        }