Linq：加載父實體和子實體

Question

我正在嘗試將SQL Server 2008數據庫中的數據加載到服務器端DTO中以傳輸到客戶端，並且想知道如何加載父級和子級實體的集合。

從模型中可以看到，我正在嘗試加載所有用戶以及在tblUsermmRole表上加入的相關UserRoles。

當我運行Linq查詢時，加載子實體時會崩潰，因為該查詢無法將單個實例加載到集合中。

我可以在單個查詢中加載這些實體，還是應該將用戶加載到集合中，然后迭代構建子AccessRoles？

模型

數據傳輸對象

Public Class User

    Public Property ID As Int32
    Public Property Username As String
    Public Property Password As String
    Public Property AccessRoles As IList(Of UserRoles)

End Class

Public Class UserRoles
    Public Property Role As ApplicationRole
End Class

Public Class ApplicationRole

    Public Property ID As Int32
    Public Property Description As String

End Class

資料載入

Dim var = (From usr In ctx.tblUsers.OrderBy(Function(w) w.username)
              Join useraccess In ctx.tblUsermmRoles On usr.idUser Equals useraccess.idUser
              Join role In ctx.tblUserRoles On role.idRole Equals useraccess.idRole
                Select New User With {.ID = usr.idUser,
                                      .Username = usr.username,
                                      .Password = usr.pwd,
                                      .AccessRoles = New ApplicationRole With {.ID = role.idRole,
                                                                               .Description = role.description}}).ToList

異常消息

{"Unable to cast the type 'Epms.Ui.Models.ApplicationRole' to type 'System.Collections.Generic.IList`1'. LINQ to Entities only supports casting EDM primitive or enumeration types."}

堆棧跟蹤

   at System.Data.Objects.ELinq.ExpressionConverter.ValidateAndAdjustCastTypes(TypeUsage toType, TypeUsage fromType, Type toClrType, Type fromClrType)
   at System.Data.Objects.ELinq.ExpressionConverter.GetCastTargetType(TypeUsage fromType, Type toClrType, Type fromClrType, Boolean preserveCastForDateTime)
   at System.Data.Objects.ELinq.ExpressionConverter.CreateCastExpression(DbExpression source, Type toClrType, Type fromClrType)
   at System.Data.Objects.ELinq.ExpressionConverter.ConvertTranslator.TranslateUnary(ExpressionConverter parent, UnaryExpression unary, DbExpression operand)
   at System.Data.Objects.ELinq.ExpressionConverter.UnaryTranslator.TypedTranslate(ExpressionConverter parent, UnaryExpression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.MemberInitTranslator.TypedTranslate(ExpressionConverter parent, MemberInitExpression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.JoinTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.Convert()
   at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Data.Entity.Internal.Linq.InternalQuery`1.GetEnumerator()
   at System.Data.Entity.Infrastructure.DbQuery`1.System.Collections.Generic.IEnumerable<TResult>.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Epms.Ui.DataProvider.DataAccess.EntityProvider.GetUsers() in C:\Users\phil.murray\Desktop\Data Provider\Epms.Ui.DataProvider\DataAccess\EntityProvider.vb:line 24
   at Epms.Ui.DataProvider.DataProvider.VB$StateMachine_0_GetUsers.MoveNext() in C:\Users\phil.murray\Desktop\Data Provider\Epms.Ui.DataProvider\DataProvider.vb:line 27

Answer 1

最后，我更改為UserRoles類以保存ID int值

Public Class UserRoles
    Public Property Role As int32
End Class

然后通過下面的Linq查詢加載數據。

Return (From usr In ctx.tblUsers.Include("tblUsermmRoles")
          Select New User With {.ID = usr.idUser,
                                .Username = usr.username,
                                .Password = usr.pwd,
                                .AccessRoles = usr.tblUsermmRoles.Select(Function(r) r.idRole)}).ToList