LINQ Query to find all tags?

Question

I have an application that manages documents called Notes. Like a blog, Notes can be searched for matches against one or more Tags, which are contained in a Note.Tags collection property. A Tag has Name and ID properties, and matches are made against the ID. A user can specify multiple tags to match against, in which case a Note must contain all Tags specified to match.

I have a very complex LINQ query to perform a Note search, with extension methods and looping. Quite frankly, it has a real code smell to it. I want to rewrite the query with something much simpler. I know that if I made the Tag a simple string, I could use something like this:

var matchingNotes = from n in myNotes
                    where n.Tags.All(tag => searchTags.Contains(tag))

Can I do something that simple if my model uses a Tag object with an ID? What would the query look like. Could it be written in fluent syntax? what would that look like?

Answer 1

For starters see my comment; I suspect the query is wrong anyway! I would simplifiy it, by simply enforcing separately that each tag exists:

IQueryable<Note> query = ... // top part of query
foreach(var tagId in searchTagIds) {
    var tmpId = tagId; // modified closures...
    query = query.Where(note => note.Tags.Any(t => t.Id == tmpId));
}

This should have the net effect of enforcing all the tags specified are present and accounted for.

Answer 2

I believe you can find notes that have the relevant tags in a single LINQ expression:

IQueryable<Note> query = ... // top part of query

query = query.Where(note => searchTags.All(st =>
    note.Tags.Any(notetag => notetag.Id == st.Id)));

Unfortunately there is no “fluent syntax” equivalent for All and Any, so the best you can do there is

query = from note in query
        where searchTags.All(st =>
            note.Tags.Any(notetag => notetag.Id == st.Id))
        select note;

which is not that much better either.

Answer 3

Timwi's solution works in most dialects of LINQ, but not in Linq to Entities. I did find a single-statement LINQ query that works, courtesy of ReSharper. Basically, I wrote a foreach block to do the search, and ReSharper offered to convert the block to a LINQ statement--I had no idea it could do this.

I let ReSharper perform the conversion, and here is what it gave me:

return searchTags.Aggregate<Tag, IQueryable<Note>>(DataStore.ObjectContext.Notes, (current, tag) => current.Where(n => n.Tags.Any(t => t.Id == tag.Id)).OrderBy(n => n.Title));

I read my Notes collection from a database, using Entity Framework 4. DataStore is the custom class I use to manage my EF4 connection; it holds the EF4 ObjectContext as a property.