Trouble with a LINQ 'filter' code throwing an error

Question

Hi folks,

I've got the following code in my Services project, which is trying to grab a list of posts based on the tag ... just like what we have here at SO (without making this a meta.stackoverflow.com question, with all due respect....)

This service code creates a linq query, passes it to the repository and then returns the result. Nothing too complicated. My LINQ filter method is failing with the following error :-

Method 'Boolean Contains(System.String)' has no supported translation to SQL.

I'm not sure how i should be changing my linq filter method :( Here's the code...

public IPagedList<Post> GetPosts(string tag, int index, int pageSize)
{
    var query = _postRepository.GetPosts()
        .WithMostRecent();

    if (!string.IsNullOrEmpty(tag))
    {
        query = from q in query
                    .WithTag(tag) // <--- HERE'S THE FILTER
                select q;
    }

    return query.ToPagedListOrNull(index, pageSize);
}

and the Filter method...

public static IQueryable<Post> WithTag(this IQueryable<Post> query,
    string tag)
{
    // 'TagList' (property) is an IList<string>
    return from p in query
           where p.TagList.Contains(tag)
           select p;
}

Any ideas? I'm at a loss :(

Answer 1

Try with Any:

public static IQueryable<Post> WithTag(this IQueryable<Post> query,
    string tag)
{
    // 'TagList' (property) is an IList<string>
    return from p in query
           where p.TagList.Any(t => t == tag)
           select p;
}

.

UPDATE (by PureKrome)

Another suggestion by Ahmad (in a comment below). This uses the Contains method so it will return all posts that contain the tag 'Test', eg. Post with Tag 'Testicle' :-

public static IQueryable<Post> WithTag(this IQueryable<Post> query,
    string tag)
{
    // 'TagList' (property) is an IList<string>
    return from p in query
           where p.TagList.Any(t => t.Contains(tag))
           select p;
}

Answer 2

In WithTag try changing the query to use a List rather than an IList:

return from p in query
       let taglist = p.TagList as List<string>
       where taglist.Contains(tag)
       select p;

Also check out this answer, which is similar to my suggestion: Stack overflow in LINQ to SQL and the Contains keyword