实体框架计数子实体

Question

我有一个实体框架查询，看起来像这样：

public object GetMatchupByVideoId(int id) 
{ 
    var videoMatchup = _DBContext.Videos 
     .Where(v => v.VideoID == id) 
     .Select(v => new { 
      id = v.MatchupID, 
      players = v.Matchup.Players.Select(mp => new 
      { 
       character = new { 
        name = mp.Character.Name, 
        votes = v.Matchup.MatchupVotes 
         .Where(mv => mv.CharacterID == mp.CharacterID) 
       }, 
       outcome = mp.Outcome 
      }) 
     }); 

    return videoMatchup; 
} 

该查询本质上给我的字符各自的投票对决。如果你看看votes属性，你会发现它仅仅根据CharacterID筛选出了选票。这工作正如我所期望的那样。

但是，我想更进一步，让每个角色的选票数量。所以，如果我改变我的查询是这样的：

public object GetMatchupByVideoId(int id) 
{ 
    var videoMatchup = _DBContext.Videos 
     .Where(v => v.VideoID == id) 
     .Select(v => new { 
      id = v.MatchupID, 
      players = v.Matchup.Players.Select(mp => new 
      { 
       character = new { 
        name = mp.Character.Name, 
        votes = v.Matchup.MatchupVotes 
         .Where(mv => mv.CharacterID == mp.CharacterID) 
         .Count() 
       }, 
       outcome = mp.Outcome 
      }) 
     }); 

    return videoMatchup; 
} 

在votes查询的末尾添加.Count()，我得到一个错误：

info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1] 
     Request starting HTTP/1.1 GET http://localhost:5000/api/videos/1/matchup application/json 
info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[1] 
     Executing action method Mindgame.Controllers.VideosController.GetMatchupByVideoId (mindgame-api) with arguments (1) - ModelState is Valid 
info: Microsoft.AspNetCore.Mvc.Internal.ObjectResultExecutor[1] 
     Executing ObjectResult, writing value Microsoft.AspNetCore.Mvc.ControllerContext. 
fail: Microsoft.AspNetCore.Server.Kestrel[13] 
     Connection id "0HL0OILHK80GT": An unhandled exception was thrown by the application. 
Microsoft.Data.Sqlite.SqliteException: SQLite Error 1: 'no such column: v.Matchup.MatchupID'. 
    at Microsoft.Data.Sqlite.Interop.MarshalEx.ThrowExceptionForRC(Int32 rc, Sqlite3Handle db) 
    at Microsoft.Data.Sqlite.SqliteCommand.ExecuteReader(CommandBehavior behavior) 
    at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, String executeMethod, IReadOnlyDictionary`2 parameterValues, Boolean closeConnection) 
    at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteReader(IRelationalConnection connection, IReadOnlyDictionary`2 parameterValues) 
    at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable.Enumerator.BufferlessMoveNext(Boolean buffer) 
    at Microsoft.EntityFrameworkCore.Query.QueryMethodProvider.<_ShapedQuery>d__3`1.MoveNext() 
    at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext() 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) 
    at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType) 
    at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType) 
    at Microsoft.AspNetCore.Mvc.Formatters.JsonOutputFormatter.WriteObject(TextWriter writer, Object value) 
    at Microsoft.AspNetCore.Mvc.Formatters.JsonOutputFormatter.<WriteResponseBodyAsync>d__9.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeResultAsync>d__32.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeResultFilterAsync>d__31.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeAllResultFiltersAsync>d__29.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeResourceFilterAsync>d__23.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeAsync>d__18.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
    at Microsoft.AspNetCore.Builder.RouterMiddleware.<Invoke>d__4.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
    at Microsoft.AspNetCore.Hosting.Internal.RequestServicesContainerMiddleware.<Invoke>d__3.MoveNext() 
--- End of stack trace from previous location where exception was thrown --- 
    at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() 
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) 
    at Microsoft.AspNetCore.Server.Kestrel.Internal.Http.Frame`1.<RequestProcessingAsync>d__2.MoveNext() 
info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2] 
     Request finished in 787.4541ms 200 application/json; charset=utf-8 

的此错误的兴趣点消息是在这里：

SQLite Error 1: 'no such column: v.Matchup.MatchupID'. 

我不确定为什么我得到这个错误，如果前面的查询工作不.Count()。此外，我在查询中完全没有使用v.Matchup.MatchupID。我只能想象这是底层SQL正在做的事情。

这里是我的模型Video，Player，“对决, and MatchupVote`：

public class Video 
{ 
    [JsonPropertyAttribute("id")] 
    public int VideoID { get; set; } 
    [ForeignKeyAttribute("Game")] 
    public int GameID { get; set; } 
    public Game Game { get; set; } 
    [ForeignKeyAttribute("Matchup")] 
    public int MatchupID { get; set; } 
    public Matchup Matchup { get; set; } 
    [ForeignKeyAttribute("User")] 
    public int UserID { get; set; } 
    public User User { get; set; } 
    public string YoutubeID { get; set; } 
    public string Description { get; set; } 
    public string Title { get; set; } 
} 

public class Player 
{ 
    [JsonPropertyAttribute("id")] 
    public int PlayerID { get; set; } 
    public int CharacterID { get; set; } 
    public Character Character { get; set; } 
    public Players.Outcomes Outcome { get; set; } 
} 

public class Matchup 
{ 
    [JsonPropertyAttribute("id")] 
    [Required] 
    public int MatchupID { get; set; } 
    public List<MatchupVote> MatchupVotes { get; set; } 
    public List<Player> Players { get; set; } 
} 

public class MatchupVote 
{ 
    [JsonPropertyAttribute("id")] 
    public int MatchupVoteID { get; set; } 
    [ForeignKeyAttribute("Character")] 
    public int CharacterID { get; set; } 
    [ForeignKeyAttribute("Matchup")] 
    public int MatchupID { get; set; } 
    public Matchup Matchup { get; set; } 
} 

所以，我的问题是，我怎么能使用.Count()方法这样得到我想要的投票计数为我查询中的每个字符？

我在这个项目中使用了.NET Core和Entity Framework Core。

Answer 1

下面是来自评论的答案，以防有人绊倒这一点。

_DBContext.MatchupVotes.Where(mv => mv.CharacterID == mp.CharacterID).Count() 

就性能而言，我不认为它会产生差异，因为底层SQL正在执行被过滤的不同计数。如果你对性能高度关注，我会检查性能分析器，但由于它是过滤计数，所以我不认为性能会受到影响。