C＃在MongoDB文档上使用投影

Question

I'm trying to get only a few values from an document that has been unwinded.

Right now I get all of this, but I only want the values from "reviews"

{ "_id" : ObjectId("57ced083857eda00e03b5a5e"), "name" : "Rest2", "reviews" : { "_id" : ObjectId("57ced083857eda00e03b5a60"), "rating" : 4, "date" : ISODate("2016-09-05T22:00:00Z") } }

My current aggregate function looks like this:

var coll = Database.GetCollection<Restaurant>("restaurants")
            .Aggregate()
            .Match(new BsonDocument { { "name", nameRest } })
            .Unwind(x => x.reviews);
        var result = await coll.ToListAsync();

How do you project only those values? Help would be greatly appreciated!

Answer 1

Based on your example, you can use project like below:

var coll = Database.GetCollection<Restaurant>("restaurants")
        .Aggregate()
        .Match(new BsonDocument { { "name", nameRest } })
        .Unwind(x => x.reviews)
        .Project(new BsonDocument { {"rating", "$reviews.rating"} })

var result = await coll.ToListAsync();

The result of the above example would output only reviews.rating and _id field. See aggregation operator $project for more information.

The snippet above was tested using MongoDB C# Driver v2.2 , MongoDB v3.2 and .Net v4.5