Question

I have 2 tables (Document and Message) that have no foreign key relation, but the table Message have the id column from Document as a column (bad design, I know..)

Table Document:
DocId,
Name

Table Message:
MessageId,
DocId,
Value

One document object can have zero or more Message objects

I want to write a query that returns a list of an aggregated object called DocumentWithMessages:

public class DocumentWithMessages
{
Document Document {get;set;}
List<Message> Messages {get;set;}
}

The query should return all Documents (with or without a list of Messages)

I started to write such a method but it gives me an exception:

public ICollection<DocumentWithDocumentSplitMessages> GetSplitJobsWithMessages()
        {
            var messages = (from dm in ObjectContext.DocumentSplitMessages select dm).ToList();
            var jobs = from d in ObjectContext.Documents                       

                       let msgs = messages.Where(dm => dm.DocID == d.DocID).ToList()

                       select new DocumentWithDocumentSplitMessages()
                           {
                               Document = d,
                               DocumentSplitMessages = msgs
                           };                                    

            return jobs.ToList();
        }

The exception is:

System.NotSupportedException : LINQ to Entities does not recognize the method ‘System.Collections.Generic.List1[BraArkivDataModel.DocumentSplitMessage] ToList[DocumentSplitMessage](System.Collections.Generic.IEnumerable1[BraArkivDataModel.DocumentSplitMessage])’ method, and this method cannot be translated into a store expression.

How do I write such a query?

Answer 1

I think you can use an approach similar to what you tried but using the IQueryable on the let clause:

var jobs = from d in ObjectContext.Documents                       

           let msgs = ObjectContext.DocumentSplitMessages.Where(dm => dm.DocID == d.DocID)

           select new DocumentWithDocumentSplitMessages()
           {
               Document = d,
               DocumentSplitMessages = msgs
           };                                    

        return jobs.ToList();