c# - Mongo update array element (.NET driver 2.0)

Question

EDIT: Not looking for the javascript way of doing this. I am looking for the MongoDB C# 2.0 driver way of doing this (I know it might not be possible; but I hope somebody knows a solution).

I am trying to update the value of an item embedded in an array on the primary document in my mongodb.

I am looking for a strongly typed way to do this. I am using the Mongodb c# 2.0 driver

I can do it by popping the element, updating the value, then reinserting. This just doesn't feel right; since I am overwriting what might have been written in the meantime.

Here is what I have tried so far but with no luck:

private readonly IMongoCollection<TempAgenda> _collection;

void Main()
{
    var collectionName = "Agenda";
    var client = new MongoClient("mongodb://localhost:27017");
    var db = client.GetDatabase("Test");
    _collection = db.GetCollection<TempAgenda>(collectionName);
    UpdateItemTitle(1, 1, "hello");
}

public void UpdateItemTitle(string agendaId, string itemId, string title){
    var filter = Builders<TempAgenda>.Filter.Eq(x => x.AgendaId, agendaId);
    var update = Builders<TempAgenda>.Update.Set(x => x.Items.Single(p => p.Id.Equals(itemId)).Title, title);
    var result = _collection.UpdateOneAsync(filter, update).Result;
}

Answer 1

Took me a while to figure this out as it doesn't appear to be mentioned in any of the official documentation (or anywhere else). I did however find this on their issue tracker, which explains how to use the positional operator $ with the C# 2.0 driver.

This should do what you want:

public void UpdateItemTitle(string agendaId, string itemId, string title){
    var filter = Builders<TempAgenda>.Filter.Where(x => x.AgendaId == agendaId && x.Items.Any(i => i.Id == itemId));
    var update = Builders<TempAgenda>.Update.Set(x => x.Items[-1].Title, title);
    var result = _collection.UpdateOneAsync(filter, update).Result;
}

Notice that your Item.Single() clause has been changed to Item.Any() and moved to the filter definition.

[-1] or .ElementAt(-1) is apparently treated specially (actually everything < 0) and will be replaced with the positional operator $.

The above will be translated to this query:

db.Agenda.update({ AgendaId: 1, Items.Id: 1 }, { $set: { Items.$.Title: "hello" } })