LINQ to SQL two contexts

Question

I'm trying to get data from two different database to retrieve data. , However; I get this error.

**The query contains references to items defined on a different data context**

    db = context1
    db2 = context2

    var query = from m in db.Merch
                join d in db.Dept on m.merchDept equals d.deptID.ToString()
                join p in db2.PROD_SKU on m.pID equals p.pID
                join f in db.FOB on d.fobCode equals f.fobID
                join w in db.Item on m.merchID equals w.merchID
                join i in db.Doc on m.merchID equals i.MerchID
                            where p.UPC_NBR.ToString() == upc

How would I go about using these two contexts to retrieve this record using linq. Any help would be appreciated.

Answer 1

Linq2Sql does not support queries on multiple database context. However, your example looks pretty simple - find p.pID by upc and then use the value as an argument in second query. It may look like this:

db = context1
db2 = context2

var upcId = (from p in db2.PROD_SKU 
            where p.UPC_NBR.ToString() == upc
            select p.pID).Single();

var query = from m in db.Merch
            join d in db.Dept on m.merchDept equals d.deptID.ToString()         
            join f in db.FOB on d.fobCode equals f.fobID
            join w in db.Item on m.merchID equals w.merchID
            join i in db.Doc on m.merchID equals i.MerchID
                        where m.pID == upcId;

Answer 2

The short answer is that you can't... not easily. In order to do a cross-database join, you have to use one Data Context that references both databases.

  <Table Name="db.dbo.Merch" Member="Merch">
    ...
  </Table>
  <Table Name="db2.dbo.PROD_SKU" Member="PROD_SKU">
    ...
  </Table>

In your DBML, manually change the names of your tables to look something like the above. You also have to make sure that you've set up permissions to allow querying both databases.

Answer 3

For a more complicated scenario, you can also execute two LINQ queries and join the results. Here I acquired the history of status updates to sales quotes and joined them with the table in another database that contains the updating users' names, so I could display them in an ItemsControl:

var firstResults = (from qsl in firstContext.QuoteStatusLogs
                    join qs in firstContext.QuoteStatus
                    on qsl.QuoteStatusID equals (qs.QuoteStatusID)
                    where qsl.QuoteID == quoteID
                    select new
                    {
                        HistoryEvent = qsl.UpdateTime,
                        QuoteStatus = qs.Description,
                        UserGUID = qsl.EmployeeGUID,
                        Comment = qsl.Comment
                    }).ToList();

var finalResults = (from r1 in firstResults
                    join e in secondContext.Employees
                    on r1.UserGUID equals (e.UserID)
                    orderby r1.HistoryEvent descending
                    select new QuoteStatusHistoryResult
                    {
                        HistoryEvent = r1.HistoryEvent,
                        QuoteStatus = r1.QuoteStatus,
                        User = e.FirstName + " " + e.LastName,
                        Comment = r1.Comment
                    }).ToList();

statusHistory = new ObservableCollection<QuoteStatusHistoryResult>(finalResults);

That first "ToList" call is important, as without it I got an exception saying I was trying to query from two contexts.