Passing ravendb query as Func<T, bool> does not work

Question

I'm playing with RavenDb and wondering if I'm missing something obvious.

Thing is, that if I'm passing query like this:

  var name = "test";
  posts = RavenSession.Query<Post>()
         .Where(x => x.Tags.Any(y => y == name))
         .OrderByDescending(x => x.CreatedAt)
         .Take(5);

It works ok, if i'm writing equivalent (IMO) using Func<T, bool>, it does not crash, but query is missing where condition:

 var name = "test";     
 Func<Post, bool> selector = x => x.Tags.Any(y => y == name);
 posts = RavenSession.Query<Post>()
         .Where(x => selector(x))
         .OrderByDescending(x => x.CreatedAt)
         .Take(5);

Profiler outputs it like:

query= start=0 pageSize=5 aggregation=None sort=-CreatedAt

Update: It works if I'm using expression instead of Func, so I thought may be I remember something wrong about Func and Linq, so wrote a simple test:

var range = Enumerable.Range(1, 50);

Func<int, bool> selector = x => x == 42;
var filtered = range.Where(x => selector(x));

So now it is only question why Raven Db query builder acts different.

Answer 1

Try using an Expression instead:

Expression<Func<Post, bool>> selector = x => x.Tags.Any(y => y == name);

And change Where(x => selector(x)) to Where(selector).

An Expression is required because RavenDb can build an expression tree from it, which allows it to translate the logic into database queries. It cannot build an expression tree from a Func<Post, bool>, so it might either ignore it, throw an exeption or whatever the creators of RavenDb specified.

Answer 2

As a response to your update, there is significant behavioral differences between Func<> on IEnumerable<> and Expression<Func<>> on IQueryable<> that look syntactically identical. This is not just Raven, but any IQueryable<> source such as LINQ to SQL or Entity Framework.