Linq : Load parent and child entities

Question

I am attempting to load data from my SQL Server 2008 database into a server side DTO to transfer over to the client and was wondering how to load collections of parent and child entities.

You can see from the model that I am trying to load all the users and the relating UserRoles which are joined on the tblUsermmRole table.

When I run the the Linq query to crashed when loading the child entities as the query can not load a single instance into the collection.

Can I load these entities in a single query or should I load the users into the collection then build the child AccessRoles iteratively?

Model

Data Transfer Object

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

Data Load

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

Exception Message

{"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."}

Stack Trace

   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

In the end I changed to UserRoles class to hold the ID int values

Public Class UserRoles
    Public Property Role As int32
End Class

Then loaded the data via the Linq query below.

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