Joe Stevens' BlogEverything .NET and C#

I’ve been looking at some of my MongoDB queries and trying to optimise them. I had one query that did a few ‘in’ statement and then sorted on a field called CreatedTime in descending order to give me the latest documents first.

My index looked something like:

{ “Field1″ : 1, “Field2″ : 1, “Field3″ : 1, “CreatedTime” -1 }

With this index I got the following information from the Explain method:

“nscanned” : 8120, “nscannedObjects” : 4364, “n” : 121, “scanAndOrder” : true, “millis” : 62

I couldn’t really work out why it was scanning my entire index, and also half the documents in the collection. Playing around with the query I found that it was the sorting that was causing it. After removing my sort I get the following result:

“nscanned” : 128, “nscannedObjects” : 126, “n” : 121, “millis” : 2

Much better, although unfortunately in the incorrect order. If I was using a capped collection I could use reverse natural order, but I’m not. I thought that as I had CreatedTime indexed descending, doing a sort on that field descending would be fine, but just for fun I thought I’d put the field as the first field specified in my index to see if that would alter how MongoDB stores and uses the index:

{ “CreatedTime” -1, “Field1″ : 1, “Field2″ : 1, “Field3″ : 1 }

Now running the query gives me the following:

“nscanned” : 256, “nscannedObjects” : 121, “n” : 121, “millis” : 2

That’s more like it, and you see the scanAndOrder attribute is no longer there. I’m limiting the query to 121 results, and the number of documents hit is 121, which is good as that means the query itself only used the index, and the exact documents were only accessed to get the fields I’m returning.

The query is scanning 256 index entries, presumably that how many it had to look at to match 121 documents based on my other query criteria.

In this post I’m going to create a simple ASP.NET MVC website for a simple blog that uses MongoDB and the offical 10gen C# driver.

MongoDB is no NOSQL database that stores information as Binary JSON (BSON) in documents. I have been working with it now for around 6 months on an enterprise application and so far am loving it. Our application is currently in alpha phase but should be public early next year! If you are used to working with an RDBMS, it takes a little bit of getting used to as generally you work with a denormalized schema. This means thinking about things quite differently to how you would previously; you’re going to have repeating data which is a no-no in a relational database, but it’s going to give you awesome performance, sure you may need an offline process that runs nightly and goes and cleans up your data, but for the real time performance gains it’s worth it.

Read the rest of this entry »

If you’re using the MongoDB C# driver with ASP.NET MVC and have a property of type ObjectId in your model you will get the following error when trying to bind back to the model.

The parameter conversion from type ‘System.String’ to type ‘MongoDB.Bson.ObjectId’ failed because no type converter can convert between these types.

This can easily be resolved by creating a custom Model Binder which I’ll explain how to do in this article.

Read the rest of this entry »

A couple of times now my MongoDB windows service won’t start after rebooting my machine. If you look at it in the services list it flicks between Starting and Started states but never really startsm and there doesn’t seem to be a way to get it to stop.

To fix this delete the mongod.lock file in your MongoDB data directory. It will immediately create a new one, but then the service will start.