How to get rank from full-text search query with Linq to SQL?

I am using Linq to SQL to call a stored procedure which runs a full-text search and returns the rank plus a few specific columns from the table Article. The rank column is the rank returned from the SQL function FREETEXTTABLE(). I've added this sproc to the O/R designer with return type Article.

This is working to get the columns I need; however, it discards the ranking of each search result. I'd like to get this information so I can display it to the user.

So far, I've tried creating a new class RankedArticle which inherits from Arti开发者_如何学Ccle and adds the column Rank, then changing the return type of my sproc mapping to RankedArticle. When I try this, an InvalidOperationException gets thrown:

Data member 'Int32 ArticleID' of type 'Heap.Models.Article' is not part of the mapping for type 'RankedArticle'. Is the member above the root of an inheritance hierarchy?

If I let the O/R designer set the sproc's own return type, it returns an int rather than a "SearchArticlesByKeywordResult" object. I'm not sure why this is, perhaps because the sproc is returning a union? Here is my procedure:

BEGIN
SET NOCOUNT ON;
(
    SELECT ftt.[rank] as [Rank], ArticleID, Subject
    FROM Article
    INNER JOIN FREETEXTTABLE( Article, (Subject, Body), @KeywordList ) AS ftt
    ON ftt.[key] = Article.ArticleID

    UNION

    SELECT ftt.[rank] as [Rank], Article.ArticleID as ArticleID, Article.Subject as Subject
    FROM Article
    INNER JOIN Solution ON Solution.ArticleID = Article.ArticleID
    INNER JOIN FREETEXTTABLE( Solution, Body, @KeywordList ) AS ftt
    ON ftt.[key] = Solution.SolutionID
)
ORDER BY [Rank] DESC
END

I can't seem to find any other questions or Google results from people trying to get the rank column, so I'm probably missing something obvious here.

---

Go to sql management studio and call your stored procedure with a @keywordlist as null. Do you see any error? Linq to SQL, in addition to other things, calls the sproc with null to discover the return columns.