Query MongoDB C#: how can I extract in a BsonDocument the field that results from calling aggregate method?

Question

I have 2 collections "images" and "user_preff".

In my app:

• "images" contains "file_name" and "data"(byte array)
• "user_preff" contains "username"(user that liked that image) and "file_name"

What I would like to do is to join this collection and I want to display the images that user liked.

I am using this one:

var documents = collection.Aggregate().Match(filter).Lookup("images", "file_name", "file_name", "common").As<BsonDocument>().Match(new BsonDocument()).ToList();

Now in the field common is what I want. How can I extract that data into another BsonDocument?

Answer 1

You can store your binary as BinData , and read the values with the following code. Projection used to select the data column and rename the field, if that's needed.

string connectionString = "mongodb://localhost:27017";
var client = new MongoClient(connectionString);
var db = client.GetDatabase("test");
//var images = db.GetCollection<BsonDocument>("images");
var userPreff = db.GetCollection<BsonDocument>("user_preff");
//images.InsertOne(BsonDocument.Parse("{\"file_name\": \"bus.jpg\", \"data\": BinData(0, \"...\")}"));
//userPreff.InsertOne(BsonDocument.Parse("{\"username\": \"ntohl\", \"file_name\": \"bus.jpg\"}"));
var filter = Builders<BsonDocument>.Filter.Empty;
var documents = userPreff.Aggregate().Match(filter).Lookup("images", "file_name", "file_name", "common")
    .Project("{\"imagesOfUser\": \"$common.data\"}").ToCursor();
while (documents.MoveNext())
{
    foreach (BsonDocument bsonDocument in documents.Current)
    {
        foreach (var image in bsonDocument["imagesOfUser"].AsBsonArray)
        {
            byte[] bytes = image.AsBsonBinaryData.Bytes;
        }
    }
}

You can get away with a simpler projection if rename is not needed:

var documents = userPreff.Aggregate().Match(filter).Lookup("images", "file_name", "file_name", "common")
    .Project("{\"_id\": 0, \"common.data\": 1}").ToCursor();
while (documents.MoveNext())
{
    foreach (BsonDocument bsonDocument in documents.Current)
    {
        foreach (var common in bsonDocument["common"].AsBsonArray)
        {
            byte[] bytes = common["data"].AsBsonBinaryData.Bytes;
        }
    }
}