Mongo C# - Where is FindOne?

Question

I'm having trouble understanding how to do a findOne operation in the Mongo C# 2.4 driver.

I have a collection with just over 9.1 million, flat documents. When I perform a findOne in Robomongo, the query takes 0 seconds, and returns the result I wanted.

In C#, this takes about 7-8 seconds.

At present I'm implementing a find one like so:

var result = await _collection.Find(filterDefinition).SingleAsync();

This takes between 7 and 8 seconds.

Mongo Query - Find One - 0 seconds

.findOne({ipFrom: { $lte: 1436497981 }, ipTo: { $gte: 1436497981 }});

Mongo Query - Find - 7.4 seconds

.find({ipFrom: { $lte: 1436497981 }, ipTo: { $gte: 1436497981 }});

I can't find a FindOne or FindOneAsync method on IMongoCollection<>, so I'm suspicious that .Find(...).SingleAsync() is just performing a mongo find function. I can see that there are FindOneAndUpdate, FindOneAndDelete, and FindOneAndReplace, but no FindOne.

My filter definition I'm using for the query is as follows:

var filter = Builders<TLocationEntity>.Filter;

var filterDefinition = filter.And(
    filter.Lte("ipFrom", ipValue),
    filter.Gte("ipTo", ipValue)
);

Basically, what's the correct way to do a findOne in the C# driver?

Answer 1

Adding Limit(1) fixed it for me:

await _collection.Find(filterDefinition).Limit(1).SingleAsync();

I would have expected .SingleAsync() to perform a Limit(1) inside it, however it seems to perform a .Limit(2) instead. This is enough to cause my query to go from 0 seconds to 8 seconds.