LINQ - EF join difficulty

Question

I have two tables:

Phase :

long ID
string Name

and another Activity :

long ID
string Name
long PhaseID

I already know the name of the phases and I want to get the activity for those particular phases. Do i add PhaseName to the activity table or do I do it through join in LINQ?

Maybe something like this?

var query = from a in entities.Activities
                        join p in entities.Phases on a.PhaseId equals p.Id
                        where p.Name == "Preplanning"

... and here im not sure how to finish this query..

Thanks for your help!

Answer 1

The code that you've provided will use an Inner Join to find all Activities where the Phase with Name "Preplanning" exists.
To finish your query you need to add a select clause.

var query = from a in entities.Activities
                 join p in entities.Phases on a.PhaseId equals p.Id
                 where p.Name == "Preplanning"
                 select a.Name   

will return IEnumerable<string> of all activity names.

Answer 2

Just select activity, as you want:

var query = from a in entities.Activities
            join p in entities.Phases on a.PhaseId equals p.Id
            where p.Name == "Preplanning"
            select a;

Here is how query expression should look like:

A query expression must begin with a from clause and must end with a select or group clause. Between the first from clause and the last select or group clause, it can contain one or more of these optional clauses: where, orderby, join, let and even additional from clauses. You can also use the into keyword to enable the result of a join or group clause to serve as the source for additional query clauses in the same query expression.

Same as puzzling image:

With method syntax you don't need to end query with something special:

var query = entities.Phases
               .Where(p => p.Name == "Preplanning")
               .Join(entities.Activities, p => p.Id, a => a.PhaseId, (p,a) => a);

Answer 3

No need to do a join if you only need data from one of the tables. You can apply a filter instead:

var q = entities.Activities.Where(a => 
        entities.Phases.Any(p => a.PhaseId == p.Id && p.Name == "Preplanning"));