How can MetaTextScore be "Injected" into a MongoDb Linq Query

Question

To full text search an IMongoQueryable<T> we can use the following extension method.

public static IMongoQueryable<T> WhereText<T>(this IMongoQueryable<T> query, string search) where T: IScored
{
   var filter = Builders<T>.Filter.Text(search);
   return query.Where(_ => filter.Inject())
}

Similarly, how can the full text search score, which is normally sorted on using :

private static readonly ProjectionDefinition<ProductTypeSearchResult> TextMatchScoreProjection =
   Builders<ProductTypeSearchResult>.Projection.MetaTextScore("Score");
private static readonly SortDefinition<ProductTypeSearchResult> Sort =
   Builders<ProductTypeSearchResult>.Sort.MetaTextScore("score");

collection.Find(...).Project(TextMatchScoreProjection).Sort(Sort);

be implemented using IMongoQueryable<T>

The examples are blank in the documentation https://mongodb.github.io/mongo-csharp-driver/2.8/reference/driver/expressions/#text

Answer 1

Given a class member:

private readonly IMongoCollection<SomeType> _someTypes;

The code to inject the query score will look like:

private IAggregateFluent<SomeType> CreateSearchQuery(string query, int skipCount, int limitCount)
{
    var filter = Builders<SomeType>.Filter.Text(query);
    return _someTypes
        .Aggregate()
        // Sadly, we must create the query every time because we don't have a way to pass in
        // a filter to an already created query, or change the properties of the filter.
        .Match(filter)
        // Currently the null coalescing operator (??) is not supported in match expressions
        // see https://jira.mongodb.org/browse/CSHARP-2708
        // Currently there is no way to "inject" the text search score
        // see https://jira.mongodb.org/browse/CSHARP-2707
        .AppendStage<SomeType>("{$addFields: {score: {$meta:'textScore'}}}")
        .Sort(_sort)
        .Skip(skipCount)
        .Limit(limitCount);
}