Question

I have 2 tables that I need to load together all the time, the both must exist together in the database. However I am wondering why Linq to Sql demands that I have to load in a collection and then do a join, I only want to join 2 single tables where a record where paramid say = 5, example…

var data = _repo.All<TheData>(); //why do I need a collection/IQueryable like this?

var _workflow = _repo.All<WorkFlow>()
                .Where(x => x.WFID== paramid)
                .Join(data, x => x.ID, y => y.WFID, (x, y) => new
                {
                    data = x,
                    workflow = y
                });

I gues then I need to do a SingleOrDefault()? If the record is not null pass it back?

I Understand the Sql query comes out correctly, is there a better way to write this?

NOTE: I need to search a table called Participants to see if the loggedonuser can actually view this record, so I guess I should leave it as this? (this is main requirement)

var participant = _repo.All<Participants>();

.Any(x=> x.ParticipantID == loggedonuser.ID); //add this to above query...

Answer 1

The line var data = _repo.All<TheData>(); is something like saying ‘start building query against the TheData table’.
This function returns you an IQueryable which will contain a definition of the query against your database.

So this doesn’t mean you load the whole TheData table data with this line!

The query will be executed the moment you do something like .Count(), .Any(), First(), Single(), or ToList(). This is called deferred execution.

If you would end your query with SingleOrDefault() this will create a sql query that joins the two tables, add the filter and select the top most record or null(or throw an error if there are more!).

You could also use Linq instead of query extension methods.
It would look like:

var data = _repo.All<TheData>(); 

var _workflow = from w in _repo.All<WorkFlow>()
                join t in _repo.All<TheData> on  w.Id equals t.WFID
                where x.WIFD = paramid
                select new
                {
                    data = t,
                    workflow = x
                });