Hey all,

This is my first post on channel9 so let me start by saying "Hi!" I hope the holiday season has been treating you all well.

Now, onto the problem I have been having with LINQ...

I am new to LINQ and I have been trying to do something pretty simple with a DB I am playing around with: There are 3 tables, User, UserToClub, and Club. UserToClub maps clubs to a given user and has the user's ID and the club's ID as FKs. When I generated EDMX for this DB I got two Entities - User and Club. I just want to filter clubs based on a given list of users, in this case based on a given city. The code compiles fine but I get a runtime error when I try to call Count on myClubs which I guess is when the statement gets actually evaluated. The error is:

 LINQ to Entities does not recognize the method 'Boolean Contains(Club)' method, and this method cannot be translated into a store expression. (System.NotSupportedException)

Any help would be appreciated. The code follows:

It seems pretty straightforward to me but I guess I am missing something (yeah, "Doh!", I know...).


Just when I was starting to really love LINQ...

ulas

var myClubs =
    from aClub in myDB.Club
    where aClub.Users.Any(aUser => aUser.City == city)
    select aClub;

This assumes that the navigation property from Club to User is named "Users" (if you haven't changed anything, it's probably "User").

dpratt71 said:

The most straightforward way I can think to do this involves stepping outside the "from...in" syntax a bit:

var myClubs =
    from aClub in myDB.Club
    where aClub.Users.Any(aUser => aUser.City == city)
    select aClub;

This assumes that the navigation property from Club to User is named "Users" (if you haven't changed anything, it's probably "User").

ulas said:

dpratt71 said:

*snip*

Ah, thanks for the reply I will give this a try in a bit and let you know what happens

Quick question though - what is the main reason why my initial version was not working? Did I miss something obvious or is this a matter of lack of support in Linq for a particular type of query? (during runtime since VS does not complain at compiletime).

ulas

Basically, it has to do with the fact that the Entity Framework is handed an object graph that represents your query. EF then has to translate that representation into a SQL query. There are many things you can do in code that EF simply cannot translate.

dpratt71 said:

The most straightforward way I can think to do this involves stepping outside the "from...in" syntax a bit:

var myClubs =
    from aClub in myDB.Club
    where aClub.Users.Any(aUser => aUser.City == city)
    select aClub;

This assumes that the navigation property from Club to User is named "Users" (if you haven't changed anything, it's probably "User").

ulas said:

dpratt71 said:

*snip*

Yup, that worked like a charm Thanks a lot!

I hope VS will be able to tell what is not supported in LINQ queries at some point. Or even better, may be the Entity Framework will be able to translate everything you can do in code

stevo_ said:

ulas said:

*snip*

We'll it can't do that (translate everything), but they do have a pattern for letting you describe a proceedure to use for a given method. 

http://blogs.msdn.com/efdesign/archive/2008/10/08/edm-and-store-functions-exposed-in-linq.aspx

It would be nice if the model was more 'stood off' I feel, meta data is great - but when you don't even control the source, its just as useless..

Even still the design is imperfect but thats the way the ecosystem is for this software..

ulas said:

stevo_ said:

*snip*

Ah, thanks for the link. I am pretty happy with the expressiveness of Linq in terms of simplifying a lot of mundane tasks one tends to do in code. It is just a bit of a downer to get everything compiling happily and then hit a runtime exception when the expression is evaluated. I realize, in a way, this is similar to trying to dividing one variable with another which may happen to be zero which would throw you a runtime exception but I just feel like checking which functions you use in the linq expression at compile time wouldn't be as difficult.