实体框架 - 查询只返回一个结果

Question

我有一个包含三个实体的应用程序：events、eventBands、bands。我在控制器类中编写了以下查询，但它似乎只返回一个结果。我希望它返回数据库中链接到该事件的所有乐队。

// EventController.cs（编辑方法）

    var viewModel = new EventUpdateData();
                    Event et = await _context.Event.Where(e => e.EventId == id)
                                                   .Include(b => b.EventBands)                                               
                                                   .FirstOrDefaultAsync();
    
                    viewModel.Event = et;
                    viewModel.EventBand = evtBand;
                    viewModel.Bands = et.EventBands.Select(b => b.Band).ToList();
    
                    return View(viewModel);

// EventUpdateData (ViewModel)

    public class EventUpdateData
    {
        public Event Event { get; set; }

        public EventBand EventBand { get; set; }

        public IEnumerable <Band> Bands { get; set; }
        
    }

// Edit.cshtml（选择字段的数据库中所有波段的列表）

            <div class="col-sm-2">
                <label for="band-list">Bands</label>
            </div>
            <div class="col-sm-5">
                <select name="band-list" id="band-list" class="band-list form-control">
                    <option value="#">--Please select a band--</option>
                    @if (Model.Bands != null)
                    {
                        @foreach (var band in Model.Bands)
                        {
                            <option value="@band.BandId">@band.BandTitle</option>
                        }
                    }
                </select>
            </div>

// Edit.cshtml（与事件相关的所有乐队的列表）

    <tbody class="bands">
    
                                    @foreach (var band in Model.Bands)
                                    {                                    
                                        var bandHours = Int32.Parse(band.BandHourlyRate) * Model.EventBand.EventBandHours;
                                    
                                        <tr class="event-band" data-bandID="@band.BandId">
                                            <td>
                                                <a href="#">@band.BandTitle</a>&nbsp;&nbsp;
                                            </td>
                                            <td class="band-rate" data-val="@band.BandHourlyRate">
                                                @bandHours
                                            </td>
                                        </tr>
                                    }
                                </tbody>

// dbo.Band（数据库）

   band_id band_title   band_contact band_phone band_rate event_id
    1   Green Fields    Mike Ellery 07110291029 100 1
    2   House Chicks    Dave Hart   07111928193 200 1
    3   Groove Surfaces Marie Penn  07566910295 150 1

（我希望查询获取所有这些波段）。

// dbo.EventBand

   EventBandId EventId BandId EventBandHourlyRate
    54  1   2   1
    55  1   1   2
    56  1   1   2
    57  1   2   1
    58  1   3   3
    59  1   1   1
    60  1   1   1
    61  1   2   2
    62  1   2   2
    63  1   1   1
    64  1   2   1

// dbo.Event（数据库）

1   Jenny's Garden Party 14/05/2021 00:00:00    Jenny Wren  The Grove
2   Bob's Balloon Party  15/06/2021 00:00:00    Bob The Garden

// Event.cs

public class Event
{
    public int EventId { get; set; }
    public string EventTitle { get; set; }

    [DataType(DataType.Date)]
    public DateTime EventDate { get; set; }
    public string EventCustomer { get; set; }
    public string EventVenue { get; set; }
    public virtual ICollection<Band> Band { get; set; }
    public virtual ICollection<EventBand> EventBand { get; set; }
    public virtual ICollection<EventCaterer> Caterer { get; set; }
}

// EventBand.cs

public class EventBand
{
    public int EventBandId { get; set; }

    public int EventBandHours { get; set; }

    public int EventId { get; set; }

    public int BandId { get; set; }

    public virtual Event Event { get; set; }

    public virtual Band Band { get; set; }
}

// Band.cs

public class Band
{
    public int BandId { get; set; }

    public string BandTitle { get; set; }

    public string BandContact { get; set; }

    public string BandPhone { get; set; }

    public string BandHourlyRate { get; set; }

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

}

Answer 1

仔细查看您的示例代码后，我认为您的映射可能会混淆 EventBands 和 Bands 的关系。为什么你同时拥有多对多（Event.EventBands）和一对多？ (Event.Bands)

EF 可以根据连接表的用途以两种方式表达多对多关系。由于您要在连接表中跟踪小时数，因此需要连接实体，它形成了更多的一对多对一关系。一个 Event 有许多 EventBand，每个 EventBand 都有一个 Band。如果依赖 EF 默认的基于约定的映射，同时拥有 EventBands 和 Bands 的集合可能会导致一些混乱。对于拥有一组 Band 的 Event，它将期望 Band 有一个 EventId。它没有，所以 EF 必须以某种方式映射关系，或者它正在沸腾到 EventBand。

使用具有 EventBandId 和额外属性的显式 EventBand 实体，您需要设置映射：

public class Event
{
    [Key]
    public int EventId { get; set; }
    // ...

    // public virtual ICollection<Band> Bands { get; set; } <- Remove.
    public virtual ICollection<EventBand> EventBands { get; set; } = new List<EventBand>();
}

public class Band
{
    [Key]
    public int BandId { get; set; }
    // ...

    public virtual ICollection<EventBand> EventBands { get; set; } = new List<EventBand>();
}

public class EventBand
{
    [Key]
    public int EventBandId { get; set; }
    [ForeignKey("Event")]
    public int EventId { get; set; }
    [ForeignKey("Band")]
    public int BandId { get; set; }
    
    public int EventBandHours { get; set; }
    public virtual Event Event { get; set; }
    public virtual Band Band { get; set; }
}

EF 应该能够毫不费力地解决此映射，但明确表示也无妨。从活动和乐队结束：

// Event config.
HasMany(x => x.EventBands)
    .WithRequired(x => x.Event); // EF 6
    //.WithOne(x => x.Event).Required(); // EF Core

// Band config.
HasMany(x => x.EventBands)
    .WithRequired(x => x.Band); // EF 6
    //.WithOne(x => x.Band).Required(); // EF Core

或者，从 EventBand 配置中：

HasRequired(x => x.Event)
    .WithMany(x => x.EventBands); 

HaRequired(x => x.Band)
    .WithMany(x => x.EventBands);

避免从两端配置关系，这可能会导致问题。要么从 Event & Band -> EventBand 映射它，要么从 EventBand -> Event & Band 映射它，而不是两者。

更新：EF 6 中关系的示例实体、映射和查询...

实体：

public class Event
{
    public int EventId { get; set; }
    public string Name { get; set; }

    public ICollection<EventBand> EventBands { get; set; } = new List<EventBand>();
}

public class Band
{
    public int BandId { get; set; }
    public string Name { get; set; }

    public ICollection<EventBand> EventBands { get; set; } = new List<EventBand>();
}


public class EventBand
{
    public int EventBandId { get; set; }
    public int Rank { get; set; }

    public virtual Event Event { get; set; }
    public virtual Band Band { get; set; }
}

关系配置：

public class EventConfiguration : EntityTypeConfiguration<Event>
{
    public EventConfiguration()
    {
        ToTable("Events");
        HasKey(x => x.EventId)
            .Property(x => x.EventId)
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);

        HasMany(x => x.EventBands)
            .WithRequired(x => x.Event)
            .Map(x => x.MapKey("EventId"));
    }
}

public class BandConfiguration : EntityTypeConfiguration<Band>
{
    public BandConfiguration()
    {
        ToTable("Bands");
        HasKey(x => x.BandId)
            .Property(x => x.BandId)
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);

        HasMany(x => x.EventBands)
            .WithRequired(x => x.Band)
            .Map(x => x.MapKey("BandId"));
    }
}

查询：

[Test]
public void TestEventBandEagerLoading()
{
    using (var context = new TestDbContext())
    {
        context.Configuration.LazyLoadingEnabled = false;
        var bands = context.Events
            .Include(x => x.EventBands)
            .Include(x => x.EventBands.Select(eb => eb.Band))
            .ToList();
    }
}

• 禁用延迟加载以证明数据是预先加载的。