Getting started with MongoDB on C# - query does not work

Question

I am starting with MongoDb on c#. At the end, I need function which simply checks if the user exists in DB -thats it. I am a complete beginner so naturally example right from MongoDb tutorial does not work here are examples:

public  static async Task<List<User>> QueryDB(User u)
{
    var collection = _database.GetCollection<User>("UserData");
    var filter = Builders<User>.Filter.Eq("id", u.id);
    var result = await collection.Find(filter).ToListAsync();
    return result;

}

or

public  static async Task<long> QueryDB(User u)
{
    var collection = _database.GetCollection<User>("UserData");
    var filter = Builders<User>.Filter.Eq("id", u.id);
    var result = await collection.Find(filter).CountAsync();
    return result;

}

What is wrong with these functions? Or how should I call them? Because now it throws a timeout. Can this be done without async/await? I think I don't need it

Answer 1

I would suggest using FindAsync, it works better and you prevent deadlocks in c# and let mongo do its job.

public async Task<List<User>> QueryDB(User u)
{
    var collection = _database.GetCollection<User>("UserData");
    var filter = Builders<User>.Filter.Eq(us => us.id, u.id); //best practice to prevent errors on field name such as extra letter or capital vs lowercase.
    List<User> fetchedUsers = new List<User>()
    using (var cursor = await collection.FindAsync(filter))
    {
          while (await cursor.MoveNextAsync())
          {
                 var batch = cursor.Current;
                  foreach (User user in batch)
                        fetchedUsers.Add(user);
          }
    }
    return fetchedUsers;

  }

if you want to "advance" you can use this method to all classes that have id field

public async Task<List<T>> QueryDB(this IMongoCollection<T> collection, T entity) where T : Idable
    {
        var filter = Builders<T>.Filter.Eq(ts => ts.id, entity.id);
        List<T> fetchedData = new List<T>()
        using (var cursor = await collection.FindAsync(filter))
        {
              while (await cursor.MoveNextAsync())
              {
                     var batch = cursor.Current;
                      foreach (T res in batch)
                            fetchedData.Add(res);
              }
        }
        return fetchedData;

      }

interface Idable
{
string id {get;};
}

public class User : Idable
{
...
}

you simply have this method "attached" to all your collections now. just create a list, lets say type of User, and call the collection.QueryDB(User u) and you will get a list of all users with the same id, yes..probably only 1, but you can modify this method and just play with it.