Linq：加载父实体和子实体

Question

我正在尝试将SQL Server 2008数据库中的数据加载到服务器端DTO中以传输到客户端，并且想知道如何加载父级和子级实体的集合。

从模型中可以看到，我正在尝试加载所有用户以及在tblUsermmRole表上加入的相关UserRoles。

当我运行Linq查询时，加载子实体时会崩溃，因为该查询无法将单个实例加载到集合中。

我可以在单个查询中加载这些实体，还是应该将用户加载到集合中，然后迭代构建子AccessRoles？

模型

数据传输对象

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

资料载入

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

异常消息

{"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."}

堆栈跟踪

   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，我更改为UserRoles类以保存ID int值

Public Class UserRoles
    Public Property Role As int32
End Class

然后通过下面的Linq查询加载数据。

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