Conversion error int? to string in Entity Framework

Question

I have problems trying to left join in a table where the PK and FK has different types. int? to string, decimal to string and vice-versa

public VDB_TABLE1()
{
   public int Id {get; set;}
   public string AnotherValue {get; set;}
   public string FKTable2 {get; set;}
   public string FKTable3 {get; set;}
}

public VDB_TABLE2()
{
   public int? PKTable2 {get; set;}
   public string AnotherValue {get; set;}
}

public VDB_TABLE3()
{
   public decimal PKTable3 {get; set;}
   public string AnotherValue {get; set;}
}

That's my ViewModel:

public VDB_MYVIEWMODEL()
{
   public int Id {get; set;}
   public string AnotherValue {get; set;}
   public VDB_TABLE2 TABLE2 {get; set;}
   public VDB_TABLE3 TABLE3 {get; set;}
}

The query:

 var query = (from t1 in context.VDB_TABLE1
                from t2 in context.VDB_TABLE2.Where(t2 => t2.PKTable2 == t1.FKTable2).DefaultIfEmpty()
                from t3 in context.VDB_TABLE3.Where(t3 => t3.PKTable3 == t1.FKTable3).DefaultIfEmpty()
 select new MYVIEWMODEL
 {
   Id = t1.Id,
   AnotherValue = t1.AnotherValue,
   TABLE2 = t2,
   TABLE3 = t3
 }).ToList();

Answer 1

It may be impossible to get EF to behave this way. MSSQL Server will not even let you make this type of relationship. If you don't mind a little SQL, you can do what you want using LinqToSQL

Modify your view model

public VDB_MYVIEWMODEL()
{
   public int Id {get; set;}
   public string AnotherValue {get; set;}
   public string T2 {get; set;}
   public string T3 {get; set;}
}

string sSQL "Select VDB_TABLE1.Id, VDB_TABLE1.AnotherValue, VDB_TABLE2.AnotherValue as T2, VDB_TABLE3.AnotherValue as T3 from VDB_TABLE1 left join VDB_TABLE2 on VDB_TABLE1.FKTable2 = VDB_TABLE2.PKTable2 left join VDB_TABLE3 on VDB_TABLE1.FKTable3 = VDB_TABLE3.PKTable3;";

List<VDB_MYVIEWMODEL> Results = context.Database.SqlQuery<VDB_MYVIEWMODEL>(sSql).ToList();

One other option would be to change your model like so.

public VDB_TABLE1()
{
   public int Id {get; set;}
   public string AnotherValue {get; set;}
   public string FKTable2 {get; set;}
   public VDB_TABLE2 Table2
   {
     get
     {
       int id = Convert.ToInt32(FKTable2);
       return context.VDB_TABLE2.Find(id);
     }
   }

   ...do similar for Table3
}

Query like so, and access the values like:

var query = (from t1 in context.VDB_TABLE1);

query.Id
query.AnotherValue
query.Table2.AnotherValue
query.Table3.AnotherValue

Warning, this is very inefficient. You are making two extra db calls for every record in your primary table. If you only have a few records...then this might be OK for you.