Question

I have related data living on two completely separate databases, and I need information from both of these databases. One of the databases lives on a MySql server and the other one lives on a MS SQL Server. Don’t ask why we have related data living on two completely different servers, it’s a long story.

From a high-level perspective, I need to join MySqlTableA to SQLServerTableB, do some complex restrictions, and possibly do some GROUP BYs and counts.

I’m trying to find a way to make joining between these two databases reasonably easy. I thought LINQ might solve my problems, but I can’t create a single context that has both MySql and SQL Server sources, as far as I know. I can put the two sources in different contexts — using Devart’s LinqConnect to create a MySql context — but LINQ doesn’t allow cross-context joins. (I tried the method described here, but it didn’t work: Simulating Cross Context Joins–LINQ/C#)

So what are my options? Is there a way to efficiently join tables on these two different database servers (though LINQ or otherwise), or am I going to have to loop through and join the data by hand?

EDIT:

As mentioned, I’ve already tried the AsQueryable() workaround, but I still get a cross-context exception. Here is my code:

public static MySqlDataContext mysql = new MySqlDataContext();
public static SQLDataContext sql = new SQLDataContext();

public static void Main() {
    var rows = from a in mysql.tableA
               join b in GetTableBs() on a.col equals b.col
               select a;

    //exception gets thrown when rows is enumerated.
    //InvalidOperationException: "The query contains references to items defined on a different data context."
    foreach(var row in rows) {
        ...
    }
}

public static IEnumerable<TableB> {
    return sql.TableBs.AsQueryable();
}

Answer 1

I have performed a test using latest dotConnect for MySQL and Entity Developer for SQL Server and succeeded in implementing the workaround as in the following example:

  var join = from d in GetDepts()
             from e in db1.Emps
             select new {
               e.ENAME,
               d.DNAME
             };
  join.ToList();
}
public IEnumerable<DEPT> GetDepts() {
  return db.DEPTs.AsQueryable();
}

Could you please send us a small test project illustrating the problem?
Update.The previous “cross-join” solution is actually not very appropriate.
The better solution is to materialize both collections and then perform an in-memory join of materialized objects:

public static MySqlDataContext mysql = new MySqlDataContext();
public static SQLDataContext sql = new SQLDataContext();

public static void Main() {
  var qA = mysql.tableA.ToList();
  var qB = sql.TableBs.ToList();
  var rows = from a in qA
             join b in qB on a.col equals b.col
             select a;

  foreach(var row in rows) {
      ...
  }
}