MongoDB Text Search with projection

Question

Using MongoDB with C# and driver 2.0, I am trying to do the following:

• Text search
• Sort the hits by text search score
• Project BigClass to SmallClass
  

Here is a (simplified version of) the classes:

class BigClass
{
    [BsonIgnoreIfDefault]
    public ObjectId             _id                 { get; set; }
    public string               Guid                { get; set; }
    public string               Title               { get; set; }
    public DateTime             CreationTime        { get; set; }
    // lots of other stuff

    [BsonIgnoreIfNull]
    public double?              TextMatchScore      { get; set; }       // Temporary place for the text match score, for sorting
}

class SmallClass
{
    [BsonIgnoreIfDefault]
    public ObjectId             _id                 { get; set; }
    public string               Title               { get; set; }

    [BsonIgnoreIfNull]
    public double?              TextMatchScore      { get; set; }       // Temporary place for the text match score, for sorting
}

If I do a text search, it is pretty straightforward:

var F = Builders<BigClass>.Filter.Text("text I am looking for");
var Result = MongoDriver.Find(F).ToListAsync().Result;

If I want to sort by the score of the text search, it's a bit more messy (and very POORLY documented):

var F = Builders<BigClass>.Filter.Text("text I am looking for");
var P = Builders<BigClass>.Projection.MetaTextScore("TextMatchScore");
var S = Builders<BigClass>.Sort.MetaTextScore("TextMatchScore");
var Result = MongoDriver.Find(F).Project<BigClass>.Sort(S).ToListAsync().Result;

Essentially it requires me to add a field in the class (TextMatchScore) to hold the result.

If I want to get the data, without sorting and project it to SmallClass, it is straightforward:

var F = Builders<BigClass>.Filter.Text("text I am looking for");
var P = Builders<BigClass>.Projection.Include(_ => _.id).Include(_ => _.Title);
var Result = MongoDriver.Find(F).Project<SmallClass>(P).ToListAsync().Result;

Now if "I want it all", that's where problem arises:

var F = Builders<BigClass>.Filter.Text("text I am looking for");
var P = Builders<BigClass>.Projection.MetaTextScore("TextMatchScore").Include(_ => _.id).Include(_ => _.Title).Include(_ => _.TextMatchScore);
var S = Builders<BigClass>.Sort.MetaTextScore("TextMatchScore");
var Result = MongoDriver.Find(F).Project<SmallClass>.Sort(S).ToListAsync().Result;

I get an exception:

Message = "QueryFailure flag was true (response was { \"$err\" : \"Can't canonicalize query: BadValue must have $meta projection for all $meta sort keys\", \"code\" : 17287 })."

As expected, the error is not documented anywhere as the Mongo guys expect users to self-document everything.

If I make the projection to 'BigClass', there is no problem, the code runs and just fills in the right fields.

If you google that text with C#, the posts you find are mine when I was trying to figure out the text search, which is also poorly documented.

So when we combine projection, text search and sorting, there doesn't seem to be any example anywhere and I just can't get it to work.

Does anyone know the reason for that problem?

Answer 1

This works for me:

var client = new MongoClient();
var db = client.GetDatabase("test");
var col = db.GetCollection<BigClass>("big");
await db.DropCollectionAsync(col.CollectionNamespace.CollectionName);

await col.Indexes.CreateOneAsync(Builders<BigClass>.IndexKeys.Text(x => x.Title));

await col.InsertManyAsync(new[]
{
    new BigClass { Title = "One Jumped Over The Moon" },
    new BigClass { Title = "Two went Jumping Over The Sun" }
});


var filter = Builders<BigClass>.Filter.Text("Jump Over");
// don't need to Include(x => x.TextMatchScore) because it's already been included with MetaTextScore.
var projection = Builders<BigClass>.Projection.MetaTextScore("TextMatchScore").Include(x => x._id).Include(x => x.Title);
var sort = Builders<BigClass>.Sort.MetaTextScore("TextMatchScore");

var result = await col.Find(filter).Project<SmallClass>(projection).Sort(sort).ToListAsync();

I removed the include of the TextMatchScore. It still comes back, because it was included by the MetaTextScore("TextMatchScore").

Documentation is a work in progress. We tackle the major use cases first as those hit the most people. This use case isn't that common and hasn't been documented. We certainly accept pull requests, both for code and documentation. Also, feel free to file a documentation ticket at jira.mongodb.org under the CSHARP project.