$meta (expression operator)（表达式运算符）

The C# examples on this page use the sample_mflix database from the Atlas sample datasets. 本页上的C#示例使用Atlas示例数据集中的sample_mflix数据库。To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB .NET/C# Driver documentation.要了解如何创建免费的MongoDB Atlas集群并加载示例数据集，请参阅MongoDB .NET/C#驱动程序文档中的入门。

The following Movie class models the documents in the sample_mflix.movies collection:以下Movie类对sample_mflix.movies集合中的文档进行建模：

public class Movie
{
    public ObjectId Id { get; set; }

    public string Plot { get; set; }
    
    public List<string> Genres { get; set; }
    
    public int Runtime { get; set; }
    
    public List<string> Cast { get; set; }
    
    public string Title { get; set; }

    [BsonElement("lastupdated")]
    public DateTime LastUpdated { get; set; }
    
    public string Rated { get; set; }

    public int Year { get; set; }
    
    public ImdbData Imdb { get; set; }

    public string Type { get; set; }

    public int Index { get; set; }
    
    public string[] Comments { get; set; }

    public List<BsonDocument> Highlights { get; set; }
    
    public float Score { get; set; }

    [BsonElement("scoreDetails")]
    public SearchScoreDetails ScoreDetails { get; set; }
    
    [BsonElement("searchScoreDetails")]
    public SearchScoreDetails SearchScoreDetails { get; set; }
    
    [BsonElement("paginationToken")]
    public string PaginationToken { get; set; }
    
    [BsonElement("plot_embedding")]
    public float[] PlotEmbedding { get; set; }
}

var camelCaseConvention = new ConventionPack { new CamelCaseElementNameConvention() };
ConventionRegistry.Register("CamelCase", camelCaseConvention, type => true);

To include the text search score by using the .NET/C# driver, call the MetaTextScore() method on the projection builder.

First, run the following code to create a text index on the Title field:

var indexModel = new CreateIndexModel<Movie>(
    Builders<Movie>.IndexKeys.Text(m => m.Title));
movieCollection.Indexes.CreateOne(indexModel);

Then, add the following code to your application. The code performs a text search for the term "future" and uses the MetaTextScore() method to include the score assigned to each matching document. The code then sorts the results in descending order by their text score and returns the document with the highest score.

var filter = Builders<Movie>.Filter.Text("future");
var projection = Builders<Movie>.Projection
    .Include(m => m.Title)
    .Include(m => m.Plot)
    .MetaTextScore(m => m.Score);

var result = movieCollection.Find(filter)
    .Sort(Builders<Movie>.Sort.MetaTextScore("score"))
    .Project(projection)
    .FirstOrDefault();

The preceding operation returns the following document:上述操作返回以下文档：

{
  "_id": {
    "$oid": "573a1398f29313caabceb500"
  },
  "plot": "After visiting 2015, Marty McFly must repeat his visit to 1955 to
   prevent disastrous changes to 1985... without interfering with his first trip.",
  "title": "Back to the Future Part II",
  "score": 1.59375
}