Retrieve Relevance Ordered Result from Text Query on MongoDB Collection using the C# Driver

Question

I am attempting to text query a collection and retrieve the results in the text match order. The docs explain pretty well how to do this in the shell:

db.articles.find(
   { status: "A", $text: { $search: "coffee cake" } },
   { score: { $meta: "textScore" } }
).sort( { date: 1, score: { $meta: "textScore" } } )

but it requires the projection of the additional score field from the find into the sort.

In C#, I have a function that looks like this:

public IEnumerable<T> TextSearch<T>(MongoCollection<T> coll, string text) {
    var cursor = coll.Find(Query.Text(text))
        .SetSortOrder(SortBy<T>.MetaTextScore(???));
    foreach(var t in cursor) {
        // strip projected score from value
        yield return t;
    }
}

but I am missing how to project the "textScore" value into my results, so that I can specify the column to MetaTextScore in SetSortOrder.

Answer 1

I was able to get this working through trial and error. The trick is that your data object needs to have a field on it already that will hold the MetaTextScore value. So given the interface:

interface ITextSearchSortable {
    double? TextMatchScore { get; set; }
}

the final function looks like this:

public IEnumerable<T> TextSearch<T>(MongoCollection<T> coll, string text) where T:ITextSearchSortable {
    var cursor = coll.Find(Query.Text(text))
        .SetFields(Fields<T>.MetaTextScore(t => t.TextMatchScore))
        .SetSortOrder(SortBy<T>MetaTextScore(t => t.TextMatchScore));
    foreach(var t in cursor) {
        // prevent saving the value back into the database
        t.TextMatchScore = null;
        yield return t;
    }
}

It's worth noting that TextMatchScore can't have a [BsonIgnore] decoration, or there will be an exception. However, it can have a [BsonIgnoreIfNull] decoration. So by scrubbing the value off the data object before yielding it, the data object can be saved back into the collection without putting in a garbage value.